Linear relationships have been established between the dissociation enthalpies of the monohydrates of Li+, Na+, K+, Be2+, Mg2-, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Al3+, Sc3+, Ti3+, V3+, Mn3+, Fe3+ and Ga3+ calculated at the MP2(FULL)/6-311++G**//MP2(FULL)/6-311++G** level, the hexahydrates of Na+, K+, Mg2+, Ca2+, Mn2+, Zn2+, Al3+, Sc3+, Fe3+ and Ga3+ calculated at the MP2(FULL)/6-311++G**//HF/6-311++G** level, and experimentally reported pK(a) values for the first ionization of the corresponding aqua-ions. In contrast to the results of Chang and Wang [Chem. Phys. Lett. 286 (1998) 46], we find that the 3+ ion hexahydrates are more (not less) stable than the 2+ ion hexahydrates. (C) 2002 Elsevier Science B.V. All rights reserved.