Reactions of gases with an atomically clean silicon surface were examined by ultrahigh-vacuum reflection electron microscopy. The initial stages of interaction of oxygen with the Si(111) surface were studied in the temperature range from 500 to 900°C. Motion of monatomic steps to the upper terraces during etching by oxygen at high temperatures was visualized. The conditions for the formation of surface vacancies were determined. The dependence of the step velocity on the width of adjacent terraces was measured. Oscillations of the intensity of the electron beam reflected specularly from the Si surface were observed during the etching of silicon by molecular oxygen, which proceeded by a two-dimensional-island mechanism. The activation energy for the diffusion of surface vacancies, which are formed owing to the interaction of oxygen with silicon, was estimated to be 1.35 ± 0.15 eV.