Molecular dynamics simulations of the pure and doped surfaces of gamma-Al2O3 were performed. Thermal behavior of both the D-layer of the (110) surface and the E-layer of the (001) surface was examined. An abrupt increase in mobility of surface ions was observed at high temperatures. An onset of diffusion occurs for the pure (110) surface at 1200 K. The instability is caused by the cation vacancies adjacent to the surface. Silicon and cerium ions deposited into the sub-surface vacancies reduce the mobility of the surface ions and prevent the onset of diffusion, cerium being more efficient in stabilization than silicon. This forms a microscopic picture of the role of additives in the stabilization of the gamma-Al2O3 surfaces.