Gas-phase binding energies and enthalpies are reported for small M(+)(H2O)(n) clusters consisting of an alkali metal cation (Li+, Na+, K+, Rb+, or Cs+) with one to six water molecules. Ab initio molecular orbital calculations were performed at the RHF and MP2 levels of theory with split valence basis sets (3-21G, 6-3+G* with effective core potentials for the heavier alkali metals). Comparison with higher level calculations and with experimentally measured bond dissociation energies suggests that the RHF/6-31+G* method provides a reasonable description of cation-water interactions in the smallest (n = 1-3) clusters. Larger clusters, particularly those that involve water-water hydrogen-bonding interactions, require a correlated treatment at the MP2 level. This study serves to calibrate the RHF/6-31+G* and MP2/6-31+G* methods for applications to cation-ligand interactions in more extended systems (e.g., the ion-selective binding of crown ethers and cryptands) for which calculations at higher levels of theory are not currently feasible.