The thermal oxidation of InP in dry oxygen has previously been shown to result in mixed oxide layers, even when grown at pressures as high as 500 atm. This appears to be caused by the slow diffusion of P and O2through the oxide relative to In. This paper reports on the oxidation of InP in steam at one and 500 atm. Growth in one atm yields an oxide with a composition very similar to those grown in dry oxygen, indicating similar growth kinetics. However, at 500 atm of steam the growth kinetics are changed dramatically and result in a uniform InPO4layer, suggesting that at high pressure, the H2O diffuses rapidly to the oxide‐InP interface where the oxidation reaction occurs.