Platinum (Pt) dissolution during oxidation/reduction cycle in various potential cycle ranges and in various pHs of the oxygen-saturated and deaerated H2SO4 and HClO4 electrolyte solutions was monitored by the highly sensitive electrochemical quartz crystal microbalance (EQCM) system. It was found that the Pt mass reduction depended on the potential cycle range and pH of the electrolyte solution with dissolved oxygen. Based on these results, it can be suggested that the dissolution of Pt requires one proton per one Pt and that it also requires dissolved oxygen, and then we can conclude that the dissolution reaction of Pt is as followings; PtO + H+ + 1/2O(2) + H2O -> Pt(OH)(3)(+) and/or PtOH + H+ + 3/4O(2) + 1/2H(2)O -> Pt(OH)(3)(+).