This paper has been written to provide a framework for the observed facts relating to the reactions between platinum and oxygen at temperatures up to the melting point. A thin film of feebly -stable solid platinum oxide is believed to form on platinum surfaces in air or oxygen at room temperatures and to thicken as the temperature is raised to about 500°C, when it decomposes. The loss of weight of platinum at higher temperatures is attributed to the formation of gaseous PtO2, and deposition of platinum on cooler surfaces (above about 500°C) to its disproportionation. A succeeding paper will deal with the oxidation of the other platinum metals.