The addition of a few mole per cent of or to the oxidizing atmosphere has been found to significantly improve the electrical stability of dry‐grown films. The results reported here were obtained with 30 min oxidation at 1150°C. The process not only decreases the mobile ion contamination originating from the furnace tube, but to a large extent also passivates the films against ionic instabilities caused by contaminated metallization. The use of or apparently also reduces the number of surface states at the interface. No significant change between standard and or oxides was observed in oxide charge, dielectric strength, dielectric constant and index of refraction, but the oxidation rate of Si is considerably increased in the presence of or . The mixture of and dry was also found to be very effective for the “cleaning” of quartz furnace tubes.