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How to Find CAS Registry Numbers
for Chemical Compounds

What is the CAS Registry Number? Why do I need it?

Chemical Abstracts Service (CAS) has a registry system for all completely identified chemical compounds or substances. There are over 128 million chemical substances & sequences currently registered and 12,000 new substances are added every day!! Each individual chemical substance is assigned a CAS Registry Number which may be thought of as that substance's "Social Security number."  (For more information, see What does a CAS Registry Number look like?) The rules of chemical nomenclature frequently change and each chemical substance is liable to have several names: i.e. trade or brand name(s); generic or common name(s); trivial or semisystematic name(s); and systematic or IUPAC name(s). For example, Tylenol is a brand name for acetaminophen which is itself a trivial name. The systematic name for this analgesic compound is Acetamide, N-(4-hydroxyphenyl)-. Another systematic name for this same compound is 4'-Hydroxyacetanilide. There is just one CAS registry number which is 103-90-2. Even if other names are created for this compound, the registry number will remain unchanged. There is no structural significance to the registry number - it is simply an identifier.

The most efficient and complete way to search for chemical substances in the Chemical Abstracts Online database is to use the CAS registry number(s).  A registry number may be searched as if it were a word, i.e., s 103-90-2 will search for information on Tylenol.  See Using Native Commands to Search Chemical Abstracts Online for more information on searching techniques.

CAS registry numbers are found in several print and electronic sources. These sources should be consulted before you develop your search strategy for Chemical Abstracts Online. It is a good idea to verify registry numbers found in sources not published by CAS. This is because the registry number could represent some other form of the compound (e.g., stereoisomer, salt, or an incompletely defined form) than the one you want. To verify the registry number, select the Registry File by typing  file reg  into the search box in Chemical Abstracts Online.  Enter s ###-##-# into the search box (###-##-# is the registry number - fill in the digits for the compound of interest). Check the record retrieved and make certain it is the desired substance before you use it in searching Chemical Abstracts Online.

If the substance retrieved is not the one you want or you cannot find the CAS registry number in any of the sources listed below, you may then search the Registry File by substance name or molecular formula. A structure search may be attempted if all else fails but this type of search is expensive and can be complicated.

What sources contain CAS Registry Numbers?