Chlorine has been incorporated into SiO2films by thermal oxidation of Si in a mixture of O2and HCl gases. Mobile sodium ions adsorbed on the oxide surface were drifted to the Si‐SiO2interface (0.5 MV cm−1bias at 200 °C). Passivation, defined as the fraction of mobile sodium charge neutralized, exhibited a pronounced threshold with HCl content in the growth ambient. This was partly due to a similar variation of oxide chlorine content (measured by &agr;‐particle backscattering). For fixed growth temperature, passivation was a monotonic function of oxide chlorine content, and was only weakly dependent on the level of sodium contamination over the range 5×1011to 1×1013Na/cm2.