The supersaturation of interstitials during oxidation in pyrogenic steam at 900 and 1000 °C was determined through measurements of enhanced phosphorus diffusion. At 900 °C, the supersaturation during steam oxidation was found to be significantly less than that observed during dry oxidation at the same growth rate, and the interstitial supersaturation varied as the square root of the oxidation rate as predicted in previous work. At 1000 °C, interstitial supersaturation was similar to that observed in dry O2, and the interstitial concentration showed only a 0.25 power dependence on oxidation rate. It was also observed that the inert‐ambient annealing of an oxide deposited at low temperature resulted in enhanced phosphorus diffusion.