The depth distribution of phosphorus in silicon dioxide formed by reacting phosphine and silane with oxygen at 480°C, has been measured with Auger spectroscopy using (i) ramp‐etched silicon dioxide films and (ii)in situion milling. Quantitative Auger analysis showed the phosphorus concentration to be 0.5 at.% throughout most of the oxide. However, when grown directly on silicon, a phosphorusrich oxide, [sine wave] 30 Å thick and containing > 3 at.% phosphorus, was first deposited, followed by a phosphorus‐depleted region, [sine wave] 150 Å thick; both regions formed within the first minute of deposition and probably resulted from a rapid initial reaction of phosphine with the silicon surface. Neither region is formed when the doped oxide is deposited on silicon dioxide, and undoped oxides < 200 Å thick were successfully used to eliminate the phosphorus‐rich layer. There was negligible diffusion(≲100  Å )of phosphorus from a doped oxide containing 0.5 at.% phosphorus into steam oxide after heating for 30 min at 1000°C.