The University of Texas

Finding and Using Chemical Abstracts Registry Numbers

Chemical Abstracts Service has registered millions of chemical substances 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. The RN 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 a compound. If you have a RN in hand, use it as a search term in SciFinder, in place of a chemical name. Most non-bibliographic chemical databases also allow searching by Registry Number.


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


Perry-Castañeda Library
101 East 21st St.
Austin, TX. 78713

Phone: (512) 495-4350

Connect with UT Libraries

Facebook Twitter Instagram Tumblr Google Plus Flickr Pinterest YouTube

© The University of Texas at Austin 2016   UTDIRECT