In this article, we describe oxidation of molten Sn–0.07 wt% P alloy exposed in air at 280 °C. Although oxidation can be effectively reduced by the addition of trace phosphorus, the oxide film has a poor protection for the molten alloy with a high P concentration. The oxide film was analyzed by x-ray photoelectron spectroscopy. Comparing the results with the low P sample, there was a high phosphorous content in the film and a new Sn–P compound on the surface of the high P sample. It is assumed that the poor protection against oxidation of the film could be due to volatility of phosphorus in the Sn–P alloy at the temperature.