CAS REGISTRY®
Access the CAS REGISTRY, the world’s most trusted source for accurate chemical substance information. Whether you’re a scientist, manufacturer, regulator, or data scientist, our comprehensive collection provides reliable data to support your work.
Look up a CAS Registry Number®What is a CAS Registry Number®?
Chemical compounds are described in many ways, including molecular formulas, chemical structures, generic, systematic, common, and trade names. This lack of clarity can cause frustration, delays, and even safety concerns.
A CAS Registry Number (CAS RN®) is a unique and unambiguous identifier for a specific substance that allows clear communication and, with the help of CAS scientists, links together all available data and research about that substance. Governmental agencies rely on CAS Registry Numbers for substance identification in regulatory applications because they are unique, easily validated, and internationally recognized.
Why are CAS Registry Numbers (CAS RNs®) used?
CAS Registry Numbers set the standard for identifying chemical substances since 1965.
- Each CAS RN uniquely identifies a chemical substance, avoiding confusion from multiple names.
- CAS RNs allow quick access to detailed chemical information in CAS solutions, including our open community resource, CAS Common Chemistry™.
- CAS RNs are a global standard, making international communication and collaboration easier.

Looking for CAS Registry Numbers or CA Index Names for known substances?
For common and frequently regulated chemicals, you can use CAS Common Chemistry, an open community resource with nearly 500,000 chemical substances from CAS REGISTRY.
If you’re looking for a more unique substance, simply complete and submit a CAS Registry Lookup service order form and receive results within 24 hours (excluding weekends and holidays).

What is the difference between the CAS REGISTRY and a CAS Registry Number?
CAS REGISTRY is the database of disclosed chemical substances curated from scientific literature and other sources. It contains over 290 million substances and is continuously enhanced with new substances and the most up-to-date information for substances in the collection.
CAS Registry Numbers (CAS RNs) are the unique numeric identifiers assigned to chemical substances within the CAS REGISTRY. A CAS RN designates only one substance, ensuring precise identification. They allow for clarity in research across regions and languages and allow CAS databases and solutions to link a wealth of information about specific substances.
![A digital interface displaying the chemical structure of a compound with CAS Registry Number 152342-19-3. Tabs labeled 'Reference (1)', 'Reactions (1)', and 'Supplier (1)' are visible. The compound's formula is C₂₂H₂₃N₃O, and its name is 4H-Pyrrolo[1,2-b]pyridazin-4-acetamide, N-butyl-2-phenyl-.](https://cdn.prod.website-files.com/650861f00f97fe8153979335/685577013343c6ae0554bf34_a204913df55d0aa9ac776a1c847272bf_registry-services-chemical-structure-cas-152342-19-3.avif)
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.

Want to license CAS REGISTRY data?
Improve your digital R&D workflows and ensure compliance when you license CAS REGISTRY data:
- Integrate CAS Registry Numbers (CAS RNs) into your digital R&D processes.
- Ensure your public data meets regulatory standards.
- Tap into a source with over 275 million unique chemicals.
CAS Registry ServicesSM
Explore our services to obtain CAS Registry Numbers and/or CA Index Names to include in your regulatory submissions, or consult with chemists for expert substance information.
CAS REGISTRY FAQ
What is CAS REGISTRY?
CAS REGISTRY is the most authoritative collection of disclosed chemical substance information. It covers substances identified in scientific literature from 1957 to the present, with additional substances going back to the early 1900s. It is updated daily with thousands of new substances.
How are CAS Registry Numbers assigned?
A CAS Registry Number is assigned to a substance when it enters the CAS REGISTRY database. New substances are added to CAS REGISTRY when they are identified in literature being indexed for the CAS Content Collection™ or when an entity requests a CAS Registry Number for their novel substance from CAS Registry ServicesSM. 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 nomenclatures 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 trade names, etc. A CAS Registry Number, however, is unique to one substance, regardless of how many other ways it can be described.
Government and regulatory 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?
CAS Registry Numbers are found in CAS solutions like CAS SciFinder® and CAS STNext®, CAS publications, and other sources such as:
- Governmental regulatory agency commercial chemical inventories.
- Handbooks, guides, and technical reports.
- Other printed reference works.
- Chemical catalogs.
If you want to find a CAS RN of a common substance, you can use CAS Common Chemistry. CAS Registry Services can provide more obscure CAS RN information for a fee.
Why do some substances in CAS REGISTRY have zero references? How can I learn more about these substances?
Records in CAS REGISTRY represent substances indexed for the CAS Content Collection, which is used in solutions such as CAS SciFinder and CAS STNext. 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 CAS STNext.
To learn more about a substance in CAS REGISTRY that does not have any literature or patent references, consult the Source of Registration (SR) and Class Identifier (CI) fields in CAS STNext.
How can I obtain or request the assignment of a CAS Registry Number?
CAS Registry Services offers several options for confirming and assigning CAS Registry Numbers and obtaining CA Index Names. Note that there are fees associated with these services.
Why is it important to come to CAS for CAS Registry Numbers?
While printed publications may contain outdated information, the CAS REGISTRY database is updated daily, making it the best source for accurate 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.
How is CAS RN pronounced?
While many in the scientific community pronounce it “cazern,” CAS RN is pronounced like an acronym, meaning each letter is pronounced: C-A-S-R-N.
What is a CA Index Name, and how are they related to CAS Registry Numbers?
A CA Index Name is the substance name that is displayed in document findings and is often the name retrieved when a CAS RN is searched in CAS REGISTRY. CAS leverages an extensive rule set to generate unique structure-based names for substances. A substance may have additional names, such as trivial names and trade names, that are also available in CAS solutions.