The peculiar chlorine evolution behavior of the platinum electrode was investigated by the measurement of the photocurrent vs. polarization potential curve and X-ray photoelectron spectroscopy, in comparison with that of the palladium electrode. The signs of photocurrent measured for platinum and palladium under anodic polarization were positive, suggesting that the surface oxide films formed on both metals have n-type conduction. The photo-current measured for platinum exhibited a wavy change with an increase in polarization potential, in analogy with the potential dependence of the rate of chlorine evolution reaction. An X-ray photoelectron spectroscopic study showed that the chemical composition of the surface oxide film formed on platinum changed with increasing polarization potential. Increases in Pt⁴⁺ ions, OM oxygen and zero-valent chlorine and a decrease in OH oxygen in the film with the increase in potential were observed. Therefore, the drop in chlorine evolution rate for platinum in the high potential region corresponds to the change in chemical composition of the film formed on platinum, that is, a change from PtO to PtO₂. The wavy changes in the chlorine evolution rate and the photocurrent with an increase in potential was explained by a change in potential difference at the Helmholtz layer corresponding to that in the surface state from Pt²⁺-Cl⁻ to Pt⁴⁺-Cl⁻.