The energetics and structures of water and methane interacting with the aluminum-terminated surface of (0001) α-Al2O3 was studied using ab initio calculations based on (Al2O3)n cluster models. The calculations indicated that the surface is highly ionic. Molecular and dissociative adsorption of water have similar exothermicities agreeing with recent experimental measurements. Also the OH stretch frequency and density of OH groups on the hydroxylated surface. Methane formed a strong physisorption interaction with the Al2O3 surface, arising predominantly from a strong electrostatic interaction between a surface aluminum cation and the methane carbon atom. The calculations for methane interacting with the Al2O3 clusters were used to derive an empirical potential for hydrocarbon physisorption on Al2O3 surfaces. Original is an abstract.