Experiments on reactive ion etching of plasma enhanced chemical vapor deposition (PECVD) phosphorus‐doped oxide [phosphosilicate glass (PSG)] and thermal oxide films have revealed significant differences which allow highly selective etching of PSG over SiO2. Samples were etched in a diode reactor with a Teflon‐covered cathode in a mixture of CF4and H2, and the proportion of hydrogen was varied over the whole range of 0%–100%. The plasma chemistry is significantly affected as the proportion of hydrogen in the mixture is increased leading to a transition from etching of oxide to deposition of a fluorocarbon film on the oxide surface. This transition occurs near 50% H2under our experimental conditions. In contrast to thermal oxide, PSG films with 7% phosphorus continue to etch in mixtures with as much as 80%–90% H2under the same conditions, and the etch rate is insensitive to the proportion of hydrogen. This effect is most likely due to the presence of phosphorus in the doped oxide film since PECVD borosilicate films etch similarly to thermal oxide. Furthermore, the transition from etching to deposition depends on the phosphorus concentration in the doped film.