How to do a CAS lookup and find a chemical by its CAS number

To do a free CAS lookup, paste the number into CAS Common Chemistry, PubChem or the EPA CompTox Chemicals Dashboard to get the compound name, chemical formula and structure. For EU regulatory status, search the same number in ECHA CHEM. For the EU customs code, use the Commission's ECICS database, which also gives you the CUS number.

Where to run a CAS lookup

CAS Common Chemistry

This is CAS's own free site, an "open community resource" covering nearly 500,000 substances taken from CAS REGISTRY. The content is licensed CC BY-NC 4.0. A free API is available on request through a web form, and commercial use of the API needs a separate licence.

The catch is coverage. The full registry holds around 300 million substances, so Common Chemistry is a small slice of it. If your number isn't there, that doesn't mean it's invalid. The full registry sits behind CAS's paid products, CAS SciFinder and CAS STNext.

PubChem

To find the compound name for a CAS number, type it into PubChem's main search box. Searching 50-00-0 returns formaldehyde (CID 712). On the compound page, look under Names and Identifiers, then Other Identifiers, for the CAS number. The same page gives the molecular formula, so one search answers both "compound name for CAS" and "CAS chemical formula".

PubChem also lists deprecated CAS numbers. The L-carnitine record, for example, carries 7634-98-2, a number that is "no longer used" but that "users may still encounter."

EPA CompTox and the TSCA Inventory

The CompTox dashboard searches 1,386,492 chemicals by CASRN. Its batch search takes up to 10,000 identifiers at once and checks the CASRN checksum, which helps with supplier spreadsheets.

For US TSCA status, search the non-confidential Inventory by CAS RN or name through EPA Substance Registry Services. Confidential entries show an EPA accession number and a generic name instead of a CAS RN. A CAS search alone cannot rule out an Inventory listing.

ECHA CHEM

Search ECHA CHEM by CAS number or EC number for EU substance records. The C&L Inventory is now part of it. ECHA decommissioned the old registration pages, the old C&L Inventory and Brief Profiles on 18 December 2025. Start a fresh search if an old bookmark fails.

ECICS for customs codes

ECICS, the European Customs Inventory of Chemical Substances, is run by DG TAXUD. It was first published in 1974. It links chemicals to their 8-digit Combined Nomenclature code and mainly covers pure chemicals: pharmaceuticals, pesticides, chemical weapons precursors, narcotic drugs and precursors, ozone-depleting substances and hazardous chemicals. You can search it by CAS RN, CUS number, CN code, EC number, UN number, name, InChI or InChIKey, in 24 languages. For an HS code search by CAS number, this returns the EU CN code. ECICS information is not binding.

What a CAS number is and how to check one

A CAS Registry Number is "a unique and unambiguous identifier for a specific substance." Chemical Abstracts Service (CAS), a division of the American Chemical Society, assigns the numbers in sequence as its scientists identify new substances. CAS calls itself "the source and final authority" for them.

The format is up to 10 digits in three hyphenated groups: 2 to 7 digits, then 2 digits, then one check digit. To test the check digit, drop it, multiply the remaining digits from the right by 1, 2, 3 and so on, and add them up. The last digit of the sum should match. CAS's own example is 107-07-3: 7x1 + 0x2 + 7x3 + 0x4 + 1x5 = 33, so the check digit is 3.

But a passing check digit only tells you the number is well formed. It doesn't tell you it's the right substance.

CAS vs EC vs CUS numbers

CLP Annex VI identifies the EC number (EINECS, ELINCS or NLP) as "the official number of the substance within the European Union." Its format is XXX-XXX-X. EINECS numbers start at 200-001-8, ELINCS at 400-010-9 and NLP at 500-001-0. The CAS number also appears in Annex VI "to assist identification."

In SDS Section 3, REACH Annex II, point 3.2.4 requires the EC number if available. The CAS number may also be given.

The CUS number is DG TAXUD's own identifier for chemical products. Its format is 9999999-9. The Commission calls it "an easy identifier" for searching ECICS and talking to customs.

Common traps

• Hydrates. One EINECS number covers both the anhydrous and hydrated forms, which often have different CAS numbers. Annex VI lists the anhydrous CAS number, so a search on a hydrate's CAS number can miss the harmonised entry.
• ECHA list numbers. Numbers starting with 6, 7, 8, 9 or a letter are ECHA list numbers, not EC numbers. They have no legal significance and don't belong on an SDS. ECHA is also moving them to an alphanumeric format such as A00-001-5.
• Entry matching. Check which CAS numbers an ECHA CHEM entry lists before you assume a match.
• Wrong numbers on third-party sites. CAS itself warns that "other resources may have associated an incorrect CAS Registry Number" with a compound.
• Treating ECICS as a ruling. ECICS information "is not binding." Check the classification with your customs authority before you rely on it.
• Publishing CAS numbers. CAS says an organisation needs a CAS Registry Number licence to publish CAS RNs to the public. Check the licence terms before reusing a database in a public service.

If your question goes past identity, for example whether a CAS number is on the Candidate List or what its CLP classification is, you can ask RegAffairs AI with the number and get a cited answer.