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CAS Common Chemistry is a free, openly accessible collection of nearly 500,000 chemical substances, operated by CAS, a division of the American Chemical Society. It lets researchers look up a substance by name, CAS Registry Number (CAS RN®), SMILES or InChI and confirm which compound a name or number refers to. For the wider field it serves, see what chemistry is.
Typical uses are checking a substance before ordering it, tidying a reagent list for a chemistry stockroom, or recording identifiers in an electronic lab notebook.
What It Is
CAS Common Chemistry gives public access to a subset of the CAS REGISTRY® database. When it first launched in 2009 it covered several thousand substances; in March 2021 CAS announced an expansion to nearly 500,000. The site states that it is provided under the Creative Commons CC BY-NC 4.0 license, and that using it means agreeing to that license’s terms.
Who Runs It and What It Covers
CAS (Chemical Abstracts Service) maintains the resource. According to the site, the wider CAS collection holds more than 165 million substances and is available through paid CAS products, while Common Chemistry is the openly accessible portion. Each record links a substance to identifiers such as its CAS RN® and structure information, and the site supports SMILES and InChI as search inputs.
How to Search or Use It
The search box on commonchemistry.cas.org is labelled for searching by chemical compound name, SMILES, InChI or CAS Registry Number. The site’s own examples are “aspirin”, “sodium chloride” and “58-08-2”.
- Open commonchemistry.cas.org.
- Enter a compound name, a CAS RN (with hyphens), a SMILES string or an InChI in the search box.
- Open the matching substance record and compare its name and identifiers with the one in your notes or order.
For automated lookups CAS offers a separate API. The API access page asks you to complete a form, after which a link with access information is sent to the email you provide. CAS points users who need an API for commercial use to a separate commercial offering.
Strengths and Limits
Strengths:
- Free to search, with data from the CAS REGISTRY®.
- Several search inputs (name, CAS RN, SMILES, InChI), which helps when sources use different names for the same substance.
- A stated open license, which makes reuse terms easy to check.
Limits:
- It covers nearly 500,000 substances, a small share of the more than 165 million in the full CAS collection, so a less common compound may not appear.
- The CC BY-NC 4.0 license is non-commercial, so commercial reuse needs a different arrangement with CAS.
Alternatives
For open chemical records and bioassay data, see our guide to PubChem. For preprints in the field, see ChemRxiv. The full CAS collection is a paid product from CAS.
Frequently Asked Questions
Is CAS Common Chemistry free to use?
Yes. The site is openly accessible and is provided under the CC BY-NC 4.0 license.
How does it differ from the full CAS collection?
The site describes the full CAS collection as more than 165 million substances, available through paid CAS products. Common Chemistry covers nearly 500,000.
Can I use its data in commercial software?
The license is non-commercial (CC BY-NC 4.0). The API page directs people who need an API for commercial use to a separate CAS commercial offering.
Is there an API?
Yes. Request access through the form on the CAS API access page; a link with access information is sent to your email.
What can I search by?
The search box accepts a chemical compound name, SMILES, InChI or CAS Registry Number. When you cite a record in a manuscript, you can also link related datasets with a persistent identifier such as a Crossref DOI.
Sources
- CAS. CAS Common Chemistry. https://commonchemistry.cas.org/ (checked November 7, 2026)
- CAS. Request API access for CAS Common Chemistry. https://www.cas.org/services/commonchemistry-api (checked November 7, 2026)
- CAS. CAS Common Chemistry expands collection of publicly available chemical information, March 17, 2021. https://www.cas.org/resources/press-releases/common-chemistry (checked November 7, 2026)
