We theoretically analyzed the formation energy and solvation free energy of four- and six-coordinated Pt(II) and Pt(IV) complexes with three types of ligands (H2O, OH- , and CF3SO3 -) as a model of dissolved Pt species to understand the Pt electrocatalyst degradation and dissolution mechanisms. All calculations were performed under the generalized gradient approximation (GGA) with Becke-88 exchange and Lee-Yang-Parr correlation functionals (BLYP). Solvent effects in water were estimated using the conductor-like-screening model (COSMO). The calculated results clarified that Pt(IV) complexes are more energetically favorable than Pt(II) complexes, indicating that the Pt(IV) complexes are more probable as dissolved species than Pt(II) complexes. We also analyzed the geometrical parameters (Pt⋯O) and atomic charge of O in ligands. These local relaxations about geometry and atomic charge are one of the factors to determine the stability of dissolved Pt species.