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Finding and Using Chemical Abstracts Registry Numbers

Chemical Abstracts Service has assigned over 60 million registry numbers for chemical compounds since 1965. CAS Registry Numbers (RNs) are very useful when searching for information about a specific chemical structure, as well as polymers, mixtures, alloys, and substances whose exact formula is unknown or variable. CAS has also registered tens of millions of biosequences.

A Registry Number (RN) looks like this:

where the first segment can be from two to seven digits long, followed by two digits, then a single check-digit. It is a sequential accession number from the CAS Registry database, and it carries no chemical or structural meaning in itself. It is simply an identifier for a specific substance that CAS has registered during the process of indexing the literature (or added from another source). The shorter the first segment, the older the registration and the more common (and probably better described) the compound is.

Where to Find Registry Numbers

Many printed and online reference sources about chemicals use CAS Registry Numbers as a standard identifier. (See Caveats below.)

see more Chemical Names and Structures

Using Registry Numbers as Search Terms

A Registry Number allows you to avoid using chemical names when searching for information about that compound. If you have a RN in hand, use it as a search term in SciFinder, in place of a chemical name.

(Medline [Ebsco and ISI versions] also has an RN search field. However, Medline/PubMed uses a more general RN indexing policy that is not nearly as precise as CAS' and it can cause confusing results.)

Most non-bibliographic chemical databases also allow searching by Registry Number.


Registry numbers are useful substitutes for names, but they are not perfect.

See the CAS Registry System to learn more about Registry Numbers and their importance.