The structures and predicted infrared (IR) spectra of M+ ClO3- and M+ ClO3- · H2 O, where M = Li, Na, K, Rb and Cs, have been investigated using density functional theory calculations. The structures identified in this study for Li+ ClO3-, Na+ ClO3- and K+ ClO3- ion pairs are in good general agreement with earlier experimental and theoretical investigations, notably bidentate binding of the cation by chlorate in Li+ ClO3-, two isomers of Na+ ClO3- and preferred tridentate binding of chlorate in K+ ClO3-. Our study extends the body of work to include Rb+ and Cs+ ion pairs, which both adopt the tridentate coordination geometry favored by K+ ClO3-. For M+ ClO3- · H2 O our calculations yielded multiple structures, including one not previously reported or suggested that features the H2O ligand "bridging" the metal ion and one oxygen atom of the chlorate anion. The "bridging" structure represents the global minimum for several of the hydrated ion pairs, and the importance of this conformation to the correct interpretation of matrix isolation infrared spectra of the hydrated ions pairs is discussed. © 2008 Elsevier B.V. All rights reserved.