The effects of phosphorus diffusion on the growth and shrinkage of oxidation‐induced stacking faults (OSF) has been studied in the temperature range from 950 to 1100 °C. By using Wright etch on angle‐lapped samples, OSF size dependence on depth has been obtained. OSF grow faster or shrink slower with increasing phosphorus dose, thus indicating a self‐interstitial supersaturation. Moreover, it was found that this supersaturation is not the result of phosphorus precipitation. Silicon self‐interstitial concentration normalized to its equilibrium value has been calculated from the OSF kinetics and was compared with time, phosphorus dose, and temperature. It was found that the interstitial supersaturation increases with phosphorus dose and decreases with annealing time. At the same phosphorus deposition condition, the interstitial concentration decreases with increasing temperature. Normalized self‐interstitial concentration calculated from enhanced phosphorus diffusivity at the profile tail, assuming a pure interstitialcy phosphorus diffusion mechanism, yields values approximately twice as high as those calculated from OSF kinetics. Possible reasons for this discrepancy are discussed.