Ab inirio molecular orbital calculations have been used to study the interactions of water molecules with the model sulfides CH3SCH3, CH3SCH2CO2-, and CH3SCH2CH2CO2-. A total of 21 species, including 17 complexes with up to five water molecules, were considered. in all cases geometry optimizations and frequency analyses were performed at the RHF/6-31G* computational level with no symmetry constraints. Hydration energies for all complexes are reported at the MP2/6-31(+)G**//RHF/6-31G* level, for the lowest-energy conformers of each complex at the MP-2/6-311(++)G**//RHF/6-31G* level, and for the smallest complexes at the MP2/aug-cc-pVDZ//RHF/6-31G* level. The optimized complexes CH3SCH3 . nH(2)O (n = 1, 2) are bound by long, weak, S-H hydrogen bonds. Two relative minima of the sulfide CH3SCH2CO2- were found and both involve one S-O distance which is shorter than the sum of the van der Waals radii of sulfur and oxygen, suggesting an attractive intramolecular interaction. The lowest-energy conformers of the complexes CH3SCH2CO2-. nH(2)O (n = 1-4) all involve hydrogen bonds of each water molecule with oxygen atoms of the carboxylate group, supplemented by as many hydrogen bonds among the water molecules as possible. No structure was found in which five water molecules directly interacted with the carboxylate of CH3SCH2CO2-; the fifth water molecule invariably migrated to the second coordination sphere during the optimization, leading to a structure best represented as CH3SCH2CO2-. 4H(2)O . H2(O). In order to assess the contributions of a few water molecules to the solvation energies of CH3SCH3 and CH3SCH2CO2-, enthalpy changes and free energy changes were calculated at 298 K for the complex forming reaction X + [H2O](n) --> X . nH(2)O; [H2(O)](n) is a cluster of n wafer molecules, and X is either CH3SCH3 or CH3SCH2CO2-. Thus, optimized structures for the water clusters [H2O], (n = 1-5) at the RHF/6-31G* level are also reported. The computed value of Delta G(298) for the formation of (CH3)(2)S.2H(2)O from (CH3)(2)S and a water dimer is +2.9 kcal/mol, comparable to the experimental free energy of hydration of +0.5 kcal/mol. The computed value of Delta G(298) for formation of CH(3)SCH(2)CO2-. 4H(2)O from CH3SCH2CO2- and a water tetramer is -19.4 kcal/mol, similar to 28% of the estimated bulk hydration energy. The results suggest that a significant part of the net solvation energies can be accounted for by interactions with a few water molecules but that the total solvation energy reflects weak interactions with a large number of water molecules.