CAS REGISTRY and CAS Registry Number FAQs

Some of the most frequently asked questions we receive about CAS REGISTRY® and a CAS Registry Number®.


CAS REGISTRY® is the most authoritative collection of disclosed chemical substance information.

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

CAS REGISTRY is updated daily with thousands of new substances.

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 and inorganic compounds
  • Metals and alloys
  • Minerals
  • Coordination compounds and Organometallics
  • Elements, Isotopes, and Nuclear particles
  • Proteins and nucleic acids
  • Polymers
  • Non-structurable materials and those of unknown or variable composition and biologics (UVCBs)
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, such as molecular formulas, structure diagrams, systematic names, proprietary or trade names etc.

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
  • Validated quickly and reliably
  • Internationally recognized 
Where can I find CAS REGISTRY Numbers?

Find CAS Registry Numbers in:

In CAS SciFindern , and CAS STNext® databases, CAS publications, and other sources such as:

  • Governmental regulatory agency commercial chemical inventories
  • Selected Elsevier databases
  • Handbooks, guides, and technical reports
  • Other printed reference works
  • Chemical catalogs
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 (CAS References, CAS Reactions, CAS Chemical Supplier Insights, CAS Markush) and other databases in products such as CAS SciFindern 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?

The CAS Inventory Expert Service and Nomenclature and Registration service offer you several options for confirming and assigning CAS Registry Numbers and obtaining CA Index Names. Note that there is a fee associated with these services.

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.