A CAS Number is a short string of text that refers to a chemical substance. CAS Numbers contain a sequence of up to ten numerical digits separated into three groups by two hyphens.
CAS Numbers were developed to overcome limitations of other chemical naming systems. For example, systems derived from molecular structure such as IUPAC nomenclature produce long names, involve complex language-specific naming rules, and are often difficult to use reproducibly.
CAS Numbers are assigned by Chemical Abstracts Service (CAS), a division of the American Chemical Society. The CAS Registry, a database maintained by CAS, contains the authoritative list of all CAS Numbers and their chemical associations. As of May 23, 2013 the CAS Registry contained 71.6 million substances. The most recently-assigned CAS Number was 1432018‑50‑2.
The last character of a CAS Number is a check digit derived mathematically from the preceding digits. Its purpose is to prevent human data-entry errors. Any change to the check digit or another digit has a high probability of producing an invalid CAS Number.
A CAS Number can also be referred to as a "Chemical Abstract Service Registry Number" or a "CAS RN".
CAS Registry Number is a Registered Trademark of the American Chemical Society.