Using a wide panel of quantum mechanical approaches, we have computed the structures and properties of protonated glycine-water complexes. It turns out that (1) MP2 is the minimum theoretical level to get accurate hydration energies; (2) anharmonicity strongly influences the vibrational spectra but has no impact on the complexation energy; (3) the correction of basis set superposition error significantly modifies both the energetic and entropic terms; and (4) 6-31+G(d) is the necessary minimal basis set. Using our best level of theory, we have obtained theoretical hydration enthalpies that are in perfect agreement with the most recent experimental investigations. (C) 2007 Elsevier B.V. All rights reserved.