Transport reference
UN 2521 — Diketene, stabilized
Table A of ADR 2025 and the 49 CFR 172.101 Hazardous Materials Table for this number, kept side by side and never merged, plus the CLP label elements of the substances that ship under it.
The two tables are never merged. The same UN number can carry a different packing group, a different proper shipping name and entirely different special provisions in each system. Every value below sits under the name of the jurisdiction it comes from.
01 · Two jurisdictions
UN 2521 in ADR 2025 and in 49 CFR 172.101
On the fields below the two tables agree for this number. Special provisions are deliberately absent from this comparison: identical code numbers mean different things in the two systems and are never interchangeable.
| Field | ADR 2025 Table A, Chapter 3.2 | 49 CFR 172.101 eCFR 2026-08-03 |
|---|---|---|
| Proper shipping name | DIKETENE, STABILIZED | Diketene, stabilized |
| Class or division | 6.1 | 6.1 |
| Packing group | I | I |
| Label codes | 6.1, 3 | 6.1, 3 |
| Rows in the table | 1 | 1 |
Sources: ADR 2025, Table A of Chapter 3.2 · ECE/TRANS/352 Vol. I (UNECE) · 49 CFR 172.101 Hazardous Materials Table · eCFR, revision 2026-08-03
02 · ADR 2025
UN 2521 in Table A of ADR 2025
One row in Table A of Chapter 3.2. Every value below comes from that row.
| Proper shipping name (3.1.2) | Class | Classification code | PG | Labels | Transport category | Hazard ID |
|---|---|---|---|---|---|---|
| DIKETENE, STABILIZED | 6.1 | TF1 | I | 6.1, 3 | 1 | 663 |
Source: ADR 2025, Table A of Chapter 3.2 · ECE/TRANS/352 Vol. I (UNECE). Class 6.1 is “Toxic substances” (ADR 2.1.1.1).
Transport category and the 1 000-point exemption
Column (15) of Table A decides whether a load stays under the exemption of ADR 1.1.3.6. Quantities of different categories are added with the multipliers below, and the sum must not exceed 1 000.
| Category | Max total quantity per transport unit | Multiplier in 1.1.3.6.4 | What it means |
|---|---|---|---|
| 1 | 20 | × 50 | Packing group I and the listed classes — the exemption runs out after 20 kg or litres per transport unit. |
Source: ADR 2025, 1.1.3.6.3 and 1.1.3.6.4 · ECE/TRANS/352 Vol. I (UNECE).
Special provisions — ADR 2025, Chapter 3.3
Codes from column (6) of Table A, with the text of Chapter 3.3.1. These are ADR provisions only. A code with the same number exists in 49 CFR 172.102 and means something else there — the two lists are never combined.
| SP 354 | This substance is toxic by inhalation. |
| SP 386 | When substances are stabilized by temperature control, the provisions of 2.2.41.1.21, 7.1.7, special provision V8 of Chapter 7.2, special provision S4 of Chapter 8.5 and the requirements of Chapter 9.6 apply. When chemical stabilization is employed, the person offering the packaging, IBC or tank for carriage shall ensure that the level of stabilization is sufficient to prevent the substance in the packaging, IBC or tank from dangerous polymerization at a bulk mean temperature of 50 °C, or, in the case of a portable tank, 45 °C. Where chemical stabilization becomes ineffective at lower temperatures within the anticipated duration of carriage, temperature control is required. In making this determination factors to be taken into consideration include, but are not limited to, the capacity and geometry of the packaging, IBC or tank and the effect of any insulation present, the temperature of the substance when offered for carriage, the duration of the journey and the ambient temperature conditions typically encountered in the journey (considering also the season of year), the effectiveness and other properties of the stabilizer employed, applicable operational controls imposed by regulation (e.g. requirements to protect from sources of heat, including other cargo carried at a temperature above ambient) and any other relevant factors. |
| SP 676 | For the carriage of packages containing polymerizing substances the provisions of special provision 386, in conjunction with 7.1.7.3, 7.1.7.4, 5.4.1.1.15 and 5.4.1.2.3.1, need not be applied, when carried for disposal or recycling provided the following conditions are met: (a) Before loading an examination has shown that there is no significant deviation between the outside temperature of the package and the ambient temperature; (b) The carriage is effected within a period of not more than 24 hours from that examination; (c) The packages are protected from direct sunlight and from the impact of other sources of heat (e.g. additional loads that are being carried above ambient temperature) during carriage; (d) The ambient temperatures during the carriage are below 45 °C; (e) Vehicles and containers are adequately ventilated; (f) The substances are packed in packages with a maximum capacity of 1000 litres. In assessing the substances for carriage under the conditions of this special provision, additional measures to prevent dangerous polymerization may be considered, for example the addition of inhibitors. |
Source: ADR 2025, Chapter 3.3 special provisions · ECE/TRANS/352 Vol. I (UNECE)
Carriage provisions — CV codes
| CV1 | Certain loading/unloading and public-place handling operations are prohibited for these goods. · relevant to segregation on the vehicle |
| CV13 | Any leaked/spilled substance must be cleaned up before the unit is reused. |
| CV28 | Must be segregated from foodstuffs, other consumables and animal feed (7.5.4). · relevant to segregation on the vehicle |
Source: ADR 2025, 7.5.11 · ECE/TRANS/352 Vol. II (UNECE)
Not imported and therefore not shown for this number: limited quantities (column 7a), excepted quantities (7b), packing instructions (8), tank codes (10–13) and the tunnel restriction code (15). An empty section is more honest than borrowed text.
03 · United States
UN 2521 in the 49 CFR 172.101 Hazardous Materials Table
One row in the Hazardous Materials Table.
| Symbols (1) | Proper shipping name (2) | Class (3) | PG (5) | Labels (6) | Packaging (8) | Quantity limits (9) | Vessel stowage (10) |
|---|---|---|---|---|---|---|---|
| — | Diketene, stabilized | 6.1 | I | 6.1, 3 | §173.None · §173.227 · §173.244 | Forbidden passenger Forbidden cargo | D · 25, 26, 27, 40 |
Source: 49 CFR 172.101 Hazardous Materials Table · eCFR, revision 2026-08-03. Class 6.1 is “Poisonous materials” (49 CFR 173.2). Packaging cells are section references into 49 CFR part 173: exceptions, non-bulk, bulk. Handling codes in column 10B are defined in 49 CFR 176.84, which is not part of our import — the numbers are shown as printed in the table.
Vessel stowage categories on this entry
| D | Must be stowed “on deck only” on a cargo vessel or on a passenger vessel within the 25-passenger limit; prohibited on a passenger vessel above that limit. |
Source: 49 CFR 172.101(k) · eCFR, revision 2026-08-03
Special provisions — 49 CFR 172.102
Codes from column (7) of the Hazardous Materials Table, with the text of § 172.102. These are United States provisions only and have nothing to do with the ADR codes above, even where the number is identical.
| 2 | This material is poisonous by inhalation (see § 171.8 of this subchapter) in Hazard Zone B (see § 173.116(a) or § 173.133(a) of this subchapter), and must be described as an inhalation hazard under the provisions of this subchapter. |
| 387 | When materials are stabilized by temperature control, the provisions of § 173.21(f) of this subchapter apply. When chemical stabilization is employed, the person offering the material for transport shall ensure that the level of stabilization is sufficient to prevent the material as packaged from dangerous polymerization at 50 °C (122 °F). If chemical stabilization becomes ineffective at lower temperatures within the anticipated duration of transport, temperature control is required in which case transportation is forbidden by aircraft. In making this determination factors to be taken into consideration include, but are not limited to, the capacity and geometry of the packaging and the effect of any insulation present; the temperature of the material when offered for transport; the duration of the journey and the ambient temperature conditions typically encountered in the journey (considering also the season of year); the effectiveness and other properties of the stabilizer employed; applicable operational controls imposed by regulation ( e.g., requirements to protect from sources of heat, including other cargo carried at a temperature above ambient); and any other relevant factors. |
| B14 | Each bulk packaging, except a tank car or a multi-unit-tank car tank, must be insulated with an insulating material so that the overall thermal conductance at 15.5 °C (60 °F) is no more than 1.5333 kilojoules per hour per square meter per degree Celsius (0.075 Btu per hour per square foot per degree Fahrenheit) temperature differential. Insulating materials must not promote corrosion to steel when wet. |
| B32 | MC 312, MC 330, MC 331, DOT 412 cargo tanks and DOT 51 portable tanks must be made of stainless steel, except that steel other than stainless steel may be used in accordance with the provisions of § 173.24b(b) of this subchapter. Thickness of stainless steel for tank shell and heads for cargo tanks and portable tanks must be the greater of 6.35 mm (0.250 inch) or the thickness required for a tank with a design pressure at least equal to 1.3 times the vapor pressure of the lading at 46 °C (115 °F). In addition, MC 312 and DOT 412 cargo tank motor vehicles must: a. Be ASME Code (U) stamped for 100% radiography of all pressure-retaining welds; b. Have accident damage protection which conforms with § 178.345-8 of this subchapter; c. Have a MAWP or design pressure of at least 87 psig; and d. Have a bolted manway cover. |
| B9 | Bottom outlets are not authorized. |
| T20 | Portable tank instruction. Minimum test pressure: 10 bar; minimum shell thickness (reference steel): 8 mm; pressure-relief requirements: § 178.275(g)(3); bottom opening requirements: Prohibited. See 49 CFR 172.102(c)(7)(ii), Table of Portable Tank T Codes T1-T22. |
| TP13 | Self-contained breathing apparatus must be provided when this hazardous material is transported by sea. |
| TP2 | a. The maximum degree of filling must not exceed the degree of filling determined by the following: |
| TP38 | Each portable tank must be insulated with an insulating material so that the overall thermal conductance at 15.5 °C (60 °F) is no more than 1.5333 kilojoules per hour per square meter per degree Celsius (0.075 Btu per hour per square foot per degree Fahrenheit) temperature differential. Insulating materials may not promote corrosion to steel when wet. |
| TP45 | Each portable tank must be made of stainless steel, except that steel other than stainless steel may be used in accordance with the provisions of 173.24b(b) of this subchapter. Thickness of stainless steel for portable tank shells and heads must be the greater of 6.35 mm (0.250 inch) or the thickness required for a portable tank with a design pressure at least equal to 1.3 times the vapor pressure of the hazardous material at 46 °C (115 °F). |
Source: 49 CFR 172.102 special provisions · eCFR, revision 2026-08-03
04 · Orange plate
The orange-coloured plate for UN 2521
The upper half of the plate carries the hazard identification number, the lower half the UN number. Dimensions follow ADR 5.3.2.2.1: 400 × 300 mm, black border and dividing line 15 mm, figures 100 mm high.
663 — highly toxic substance, flammable (flash-point not above 60 °C).
| 6 | Toxicity or risk of infection |
| 6 | Toxicity or risk of infection · doubled figure — intensification of that hazard |
| 3 | Flammability of liquids (vapours) and gases or self-heating liquid |
Source: ADR 2025, 5.3.2.3 · ECE/TRANS/352 Vol. II (UNECE). The tunnel restriction code of column (15) is a different value and is not part of our import.
05 · Substances
Substances shipped under UN 2521
Linked by CAS number, never by name: a name match once produced “acetone cyanohydrin” for acetone, and a wrong link here would be a wrong classification. Each entry opens its own reference page with the CLP classification and label elements.
06 · The label side
What UN 2521 looks like on a workplace label
The tables above are the transport classification. This is the other half of the same material: the CLP label elements of the substances linked to this number — what actually goes on the drum once it has been delivered. Transport marks and supply labels answer different regulations and do not replace one another.
Hazard statements on the substances under UN 2521
Source: harmonised classification, EU CLP Annex VI. Transport classification and supply classification are decided under different rules and do not have to agree.
07 · FAQ
UN 2521 — frequently asked questions
What is UN 2521?
UN 2521 is the United Nations number for Diketene, stabilized. Under ADR 2025 it is assigned to class 6.1 (toxic substances), classification code TF1.
What packing group is UN 2521?
Under ADR 2025 UN 2521 is packing group I.
Is UN 2521 classified the same way in the United States and in Europe?
On the fields we hold — proper shipping name, class, packing group and label codes — ADR 2025 and the 49 CFR 172.101 table agree for UN 2521. Special provisions still differ and are never interchangeable between the two systems.
What is the hazard identification number for UN 2521?
The orange-coloured plate for UN 2521 carries the hazard identification number 663 above the UN number. Under ADR 5.3.2.3.2 that number means: highly toxic substance, flammable (flash-point not above 60 °C).
What is the transport category of UN 2521?
Column (15) of Table A puts UN 2521 in transport category 1. Under ADR 1.1.3.6.3 the maximum total quantity per transport unit for that category is 20.
Which substances ship under UN 2521?
We hold 1 substance linked to UN 2521 by CAS number. Each one has its own reference page with the CLP classification, GHS pictograms and hazard statements that apply on the label — the workplace side of the same material.
Need the plate, the labels or the placards printed?
The orange plate above is a physical article, and so are the class labels in column (5). Our sister site GHSLabels.com covers printing and suppliers. For the workplace label of the same material, use the GHS label constructor, and browse other numbers in the UN number reference.
Reference aid only. Values are reproduced from ADR 2025 and the 49 CFR 172.101 table as published; the consignor remains responsible for the classification of a specific consignment. Always verify against the current published regulation and the supplier's safety data sheet.