The oxidation of platinum has been investigated employing electrically heated ribbons at temperatures from 900° to 1500°C and at pressures from 15 µ to 1 atm. An energy of activation of 42.5 kcal mole−1 has been obtained from the temperature‐dependence of the rate. An unusual pressure‐dependence has been observed which was found to result from the back‐reflection of the volatile by the surrounding gas molecules: At 1 atm pressure only a fraction of a per cent of the formed escapes from the platinum.