Gas phase etching of Si(111)(7 x 7) surfaces, by reaction with O2 at almost-equal-to 950 K, was directly followed by in situ STM imaging. The reaction mechanism was found to strongly depend on the density of structural defects in the respective (7 x 7) surface layer, which can act as nucleation centers for pit formation, and on the terrace size. Under present conditions we observed a step-flow mechanism for removal of the topmost, defect-free Si layer and formation of monolayer deep pits in the defective (7 X 7) layers in the further reaction process. The resulting surface is rough on an atomic scale, with typically three layers exposed.