American Chemical Society
Finding and Verifying CAS Registry Numbers® on STN®
— March 1993 —
CAS Registry Numbers
A CAS Registry Number is simply an identification code assigned by Chemical Abstracts Service to a unique substance when it is added to the Registry System. It is analogous to a Social Security number for a U.S. citizen. Only one Registry Number exists for each substance. Registry Numbers are assigned sequentially. Usually, the higher the number, the more recently it was added to the system.
The format for a Registry Number is a three part number, NNNNNN-NN-N, consisting of 2 to 6 digits, a hyphen, 2 digits, another hyphen, and 1 final digit (a check digit). As more Registry Numbers are added to the system, the number of digits in the first portion of the number will continue to grow (to 7 digits, 8 digits, etc.). Examples of valid numbers include: 543-49-7, 83541-34-8, and 127685-30-7.
Registry Number Sources
Several sources are available for finding a Registry Number:
- Online searching: the STN REGISTRY File.
- CAS Registry Services: chemists with in-depth training can find the correct Registry Number for a substance at a reasonable cost. For more information, refer to Appendix 5 — Additional Support.
- Printed Sources: refer to Appendix 8 — Printed Sources of Names and Registry Numbers.