CAS REGISTRY and CAS Registry Number FAQs




CAS REGISTRYSM is the most authoritative collection of disclosed chemical substance information, containing more than 137 million organic and inorganic substances and 67 million sequences (view current numbers).

CAS REGISTRY covers substances identified from the scientific literature from 1957 to the present, with additional substances going back to the early 1900s.

What is a CAS Registry Number (CAS RN)?



Each CAS Registry Number® (CAS RN®) identifier:

  • Is a unique numeric identifier
  • Designates only one substance
  • Has no chemical significance
  • Is a link to a wealth of information about a specific chemical substance
What does a CAS Registry Number look like?

A CAS Registry Number is a numeric identifier that can contain up to 10 digits, divided by hyphens into three parts.

The right-most digit is a check digit used to verify the validity and uniqueness of the entire number.

For example, 58-08-2 is the CAS Registry Number for caffeine.

How does CAS assign Registry Numbers?

A CAS Registry Number is assigned to a substance when it enters the CAS REGISTRY database. Numbers are assigned in sequential order to unique, new substances identified by CAS scientists for inclusion in the database.

What kinds of compounds does the CAS REGISTRY contain?

CAS REGISTRY contains a wide variety of substances, including the world's largest collection of:

  • Organic compounds
  • Inorganic compounds
  • Metals
  • Alloys
  • Minerals
  • Coordination compounds
  • Organometallics
  • Elements
  • Isotopes
  • Nuclear particles
  • Proteins and nucleic acids
  • Polymers
  • Nonstructurable materials (UVCBs)
How frequently is the CAS REGISTRY database updated?

Approximately 15,000 new substances are added each day.

See the most up-to-date CAS REGISTRY figures.

Why have CAS Registry Numbers become the world standard?

Since CAS Registry Numbers are not dependent upon any system of chemical nomenclature, they can:

  • Provide a reliable common link between the various nomenclature terms used to describe substances
  • Serve as an international resource for chemical substance identifiers used by scientists, industry and regulatory bodies
Why do regulatory agencies rely on CAS Registry Numbers?

Chemical compounds can be described in many different ways:

  • Molecular formula
  • Structure diagram
  • Systematic names
  • Generic names
  • Proprietary or trade names
  • Trivial names

A CAS Registry Number, however, is unique and specific to only one substance regardless of how many other ways the substance can be described.

Governmental agencies have found CAS Registry Numbers ideal for keeping track of substances because 

  • They are unique
  • They can be validated quickly and reliably
  • They are internationally recognized
Where can I find CAS REGISTRY Numbers?

Find CAS Registry Numbers in:

In a hurry? Contact CAS Registry Number Lookup Service and ask for 24-hour turnaround (Mon-Fri).

Why do some substances in CAS REGISTRY have zero references? How can I learn more about these substances?

Records in REGISTRY represent substances indexed for the CAS databases (CASM/CAplusSM, CASREACT®, CHEMCATS®, CHEMLIST®, MARPAT®) and other databases in products such as SciFinder® and STN®.  Substances that come from sources other than literature or patent publications may not contain any references.  CAS indexes these substances to support registration policies, chemical libraries, chemical catalogs, web sources, reaction databases and data collections not represented in STN.

To learn more about a substance in REGISTRY that does not have any literature or patent references consult the Source of Registration (SR) and Class Identifier (CI) fields in STN.

How can I obtain or request assignment of a CAS Registry Number? CAS Client Services offers you several options for confirming and assigning CAS Registry Numbers and obtaining CA Index Names. Note that there is a fee associated with this service.
Why is it important to come to CAS for Registry Numbers?

While printed publications may contain outdated information, the CAS REGISTRY database is updated daily providing your best source for the latest CAS Registry Number information.

CAS is the source and final authority for CAS Registry Numbers. Other resources may have associated an incorrect CAS Registry Number with a compound, and use of that incorrect CAS Registry Number in searches could lead you to irrelevant or inappropriate information.

What is the CAS RN Verified Partner Program? A CAS Registry Number license is required anytime an organization will “publish” CAS Registry Numbers to the public or use them to support features of a platform that is publicly or commercially available.  See the CAS Registry Number Verified Partner Program for more information.

CAS Content

Contact Us

E-mail   Phone & Fax   800-753-4227 (North America)
    614-447-3700 (Worldwide)
    614-447-3751 (Fax)