Structural and mechanistic aspects of the initial stages of gas phase etching of Si(111) - (7 x 7) surfaces, by reaction with 02 at temperatures between 910 and 1070 K, were investigated by scanning tunneling microscopy. On smaller terraces, the reaction is found to proceed via a step flow mechanism, where Si is effectively removed at the edges of retracting terraces. On very large terraces, nucleation and growth of vacancy islands contribute in addition. The data lead to a reaction mechanism involving the formation of volatile SiO on the terraces, by reaction of 0 adatoms and Si surface atoms, subsequent vacancy diffusion, and finally the annihilation of vacancies at step edges or in vacancy islands. The temperature dependence of the resulting step structures, ranging from irregular and nearly dendritic forms at approximately 910 K via faceted steps at approximately 970 K to rounded shapes above 1000 K, results from step pinning by oxide nuclei and annealing effects at higher temperatures.