The surface composition and structure of SiC (hexagonal crystallites) are studied under Ar-ion bombardment, high-temperature cycling and adsorption of oxygen. Such treatments are necessary for surface cleaning and they also simulate environments in potential applications of SiC elements. Oxygen adsorption on surfaces which result after various cycles of Ar-ion bombardment and heating, i.e., with varying surface compositions, is studied to reveal details of the initial steps of oxidation. The experimental techniques were AES, XPS, and BAES (bremsstrahlung-induced AES). Bombardment with 3-keV Ar ions results in a nearly stoichiometric (1:1) surface. The implanted Ar accumulates strongly in the subsurface region during annealing at 900 °C. Annealing at higher temperatures (1200 °C) releases the Ar but also leads to decomposition of the surface, with the loss of Si. Oxygen adsorbs on the surface, in a reaction which mainly involves silicon atoms, and with an oxygen coverage related to the initial silicon surface concentration. At room temperature, the adsorption reaction contains two stages as for oxygen on clean silicon surfaces.