Water vapor added to NF3plasma during reactive ion etching controls the ratio of the etch rates of SiO2and Si. Selectivity rises from a value of 0.14 at water‐free 100% NF3to 1.99 for an initial gas composition of 35% H2O‐65% NF3. The results of mass and energy analysis of the plasma yield a basis for discussing the mechanisms which effect the selectivity. The NF3/H2O plasma removes native oxides from Si surfaces.