Oxygen layers on platinum are dissolved at open circuit in electrolytes containing chloride ions. The dissolution of layers formed at 1.4 and 1.6v in was investigated in two solutions with different concentrations of chloride ions at 25° and 85°C, and it is shown that the dissolution occurs by the following mechanism. The electrode is the seat of simultaneous cathodic and anodic reactions, in which the sum of the partial currents of anodic platinum dissolution on the free surface is equal to the cathodic partial current of the oxygen reduction.