Molecular mechanics simulation was used to study the vacancy diffusion mechanism of impurities in corundum. Divalent and trivalent cation impurities of different sizes were investigated. The migration energies for different possible migration paths were obtained by the nudged, elastic band method. The binding energies of defect clusters responsible for impurities diffusion were also estimated. It was shown that the two paths give the main contribution to the vacancy diffusion of impurities: migration along the C axis through an aluminum interstitial site, and jumps to the nearest aluminum sites at an angle to the C axis. The migration energies of isovalent impurities weakly depend on ion size. For divalent impurities the simulation revealed that the diffusion mechanism is changed from monovacancy to divacancy transport (mediated by defect clusters containing simultaneously the oxygen and aluminum vacancies) at some concentration of oxygen vacancies. (C) 2016 Elsevier B.V. All rights reserved.