Hydration of selected platinum complexes [PtCl42-, Pt(NH3)(4)(2+), and cis- and trans-platin-PtCl2(NH3)(2)] have been studied. Up to two solvent molecules have been considered to replace the ligands. In order to be able to draw conclusions about pH changes in the course of the hydration process, both H2O and OH- species were considered in the solvating process. The modified Gaussian 3 theory was adapted for the pseudopotential treatment of platinum complexes. Since a heavy element was present in the complexes, an additional stabilization due to the spin-orbit coupling and core-polarization potentials have been evaluated above the scheme of a G3 treatment. This spin-orbit coupling stabilization amounts to 2-5 kcal/mol but does not qualitatively change the hydration preferences. In accord with the experiment, neutral Pt(NH3)(2)(OH)(2) was found to be the most stable complex for hydration of both cis- and trans-platin.