The plasma etching of silicon and silicon dioxide in CF4‐O2mixtures has been studied as a function of feed‐gas composition in a 13.56‐MHz plasma generated in a radial‐flow reactor at 200 W and 0.35 Torr. Conversion of CF4to stable products (CO, CO2, COF2, and SiF4) and the concentration of free F atoms ([F]) in the plasma were measured using a number of different diagnostics. The rate of etching, the concentration of F atoms, and the intensity of emission from electronically excited F atoms (3s2P–3p2P° transition at 703.7 nm) each exhibit a maximum value as a function of feed‐gas composition ([O2]); these respective maxima occur at distinct oxygen concentrations. For SiO2, the variation in etching rate with [O2] is accounted for by a proportional variation in [F], the active etchant. The etching of silicon also occurs by a reaction with F atoms, but oxygen competes with F for active surface sites. A quantitative model which takes oxygen adsorption into account is used to relate the etch rate to [F]. The initial increase of [F] with [O2] is accounted for by a sequence of reactions initiating with the production of CF3radicals by electron impact and followed by a reaction of CF3with oxygen. When [O2] exceeds ∼23&percent; (under the present discharge conditions), [F] decreases due, probably, to a decrease in electron energy with an increase of oxygen in the feed gas.