Abstract: This review covers the reaction mechanisms and applications of ion/molecule association reactions for solving increasingly complex analytical problems. Because these reactions are frequently fast and efficient, the use of ion/molecule reactions provides a diverse frontier for extending the boundaries of mass spectrometry. Adduct ions from ion/molecule reactions may provide the molecule mass information for those nonvolatile, labile compounds, improve the detection sensitivity, and particular product ions may afford key diagnostic information for structure identification. In addition, alkali metal ion affinities and coordination of alkali metal ion to compounds are introduced.