Reactive gas ion beams have been used to investigate mechanisms of plasma and reactive ion etching processes. The technique allows independent control of ion energy, flux and angle of incidence and background pressure. Etching of Si and SiO2 in beams generated from CF4 and Cl2 were investigated at ion energies from 100 to 1200 eV. Selective etching of SiO2 (with CF4) and Si (with Cl2) was observed as a function of ion energy, up to 1200 eV. Angular dependence of etch rates indicates that, at low energy, adsorption followed by chemical reaction is responsible for etching rather than physical sputtering.Etch yield of SiO2 with CF4 also exhibits strong dependence on background pressure as well as ion flux. The results are examined in light of reports of plasma and reactive ion etching in rf discharges of Si and SiO2 in the same gases.