Chemical substances can have several names. A CAS Registry Number provides another way to identify a substance record without relying on one spelling or naming convention. It is useful for cross-checking information, but the presence of a plausible number on a label does not verify the contents of a vial.
The identifier points to a substance record
CAS describes its Registry Number as a unique identifier for a substance in CAS REGISTRY. The number connects substance information across different names and sources. It is a numeric identifier rather than a chemical formula, so its digits are not a code from which you can read the sequence or molecular structure.CAS identifier guidance (opens in a new tab)
That distinction changes how you use it. Read the number as a key for retrieving and comparing a record. Once you find the record, inspect its substance description rather than assuming the number alone communicates all the information relevant to your sample.
A product page may also contain a catalogue number, lot number and laboratory report number. These are different identifiers maintained for different purposes. Put them in separately labelled fields so that a question about a batch is not answered with a substance identifier.
A valid-looking format is only a first check
CAS Registry Numbers use three hyphen-separated groups and include a final check digit. CAS explains that the format can contain up to ten digits. These features help with transcription checks, but a number that passes a formatting check can still have been copied against the wrong substance.CAS identifier guidance (opens in a new tab)
When copying a number from a PDF, retain it as text. Spreadsheet software can reformat identifiers, remove leading characters in other code systems or alter how long strings are displayed. The goal is an exact identifier, not a quantity to calculate with.
Compare the description behind the number
Start with the number given in the specification, then compare the returned substance name and available structural description with that specification. Pay attention to declared modifications, salt information or other identity qualifications. If the descriptions differ, investigate the difference instead of choosing whichever record looks more familiar.
CAS notes that third-party resources can associate an incorrect Registry Number with a compound. That is a reason to check the authoritative record where available, especially when two sources disagree. It is not a reason to invent a corrected number from a similar product.CAS identifier guidance (opens in a new tab)
For an original example, imagine a supplier listing a parent peptide name while another document explicitly describes a modified version. Even if both pages display the same number, that agreement does not resolve the mismatch in the molecular descriptions. Record the conflict and request clarification.
Keep substance identity separate from batch evidence
| Identifier | Question |
|---|---|
| CAS Registry Number | Which registered substance is being referenced? |
| Catalogue or product code | Which supplier listing or item is being referenced? |
| Batch or lot code | Which identified production or supplied lot is being referenced? |
| Report number and revision | Which analytical document and version is being referenced? |
Two lots can refer to the same registered substance while having different analytical reports. Conversely, a certificate displaying the expected CAS number may still describe a different lot from the vial you received. Matching the substance is useful, but it is only one part of document traceability.
Purity, content and other test results need their own evidence. A registry identifier is not a measurement, a regulatory approval or a guarantee of suitability for an experimental task. Keep those claims attached to the documents that actually support them.
Leave a traceable identity note
- Copy the Registry Number exactly as supplied.
- Record the source and the substance description you checked.
- Note disagreements or missing structural detail.
- Keep lot and report identifiers alongside it in separate fields.
The useful outcome is a transparent cross-check: this specification refers to this substance record, with these qualifications. If the record could not be accessed or a discrepancy remains, say so. A visible gap is more reliable than a confident-looking but unverified identifier.
Sources and further detail
- CAS — CAS REGISTRY and Registry Number guidance (opens in a new tab)
Authoritative explanation of Registry Numbers, format and identification role. No specific commercial substance registration was verified for this article.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.