A subject which embraces all applications of radioactive isotopes to chemistry. It is not precisely defined and is closely linked to nuclear chemistry. The widespread use of isotopes in chemistry is based on two fundamental properties exhibited by all radioactive substances. The first property is that the disintegration rate of an isotopic sample is directly proportional to the number of radioactive atoms in the sample. Thus, measurement of its disintegration rate (with a Geiger counter, for example) serves to analyze a radioactive compound. With nearly all chemical elements (the most notable exceptions being nitrogen and oxygen, which have no suitable radioactive isotopes), an isotope may be incorporated in a chemical compound, and thereafter, masses of this compound as small as 10⁻⁶ to 10⁻¹⁰ g may be measured with a high precision.

Because experimental chemistry depends largely upon analysis, isotopes may be employed in most chemical problems, especially those requiring high analytical sensitivity. The second fundamental property is that the disintegration rate is completely unaffected by the chemical form of the isotope, and conversely, the property of radioactivity does not affect the chemical properties of the isotope. By substituting or labeling a particular atom within a molecule, isotopes can be used to trace the fate of that atom during a chemical reaction. In contrast to physical migration tracer studies, the compounds arising in a reaction must first be isolated in separated pure forms before radioactive assays can be performed.