Oxidation of graphite by radiolyzed CO2 or O2 leads to the abstraction, by gaseous active species, of individual carbon atoms from basal planes producing vacancies. Vacancies and edges act as active centres for subsequent oxidation and expanding areas of carbon, 1 atom layer thick, are oxidized by a mechanism involving surface diffusion of oxygen-containing groups. The rate of vacancy formation is similar with O2 or CO2 implying that the same active species may be involved, but the rate of expansion is much higher with radiolyzed O2, suggesting different species for this process. In the presence of 1 % CH4, carbon dioxide is incapable of oxidizing graphite, and the graphite surface is modified in such a way that gold ceases to be mobile upon it at temperatures below 700°C.