The present study reports on annealing-induced interface and bulk effects in thin Cr2O3 films on aluminium oxide substrates. The samples consisted of ceramic Al2O3 substrates with 825 Angstrom Cr2O3 overlayers, deposited by reactive r.f. magnetron sputtering. Annealing in the range from 1100 degrees C to 1250 degrees C, and the subsequent AES sputter depth profiling were carried out in situ under ultra high vacuum conditions. With increasing temperature and heating time an interface broadening and a nearly constant concentration of Al developing in the Cr2O3 film are observed. On the basis of Whipple's theory for combined grain boundary and intragranular diffusion, bulk diffusion coefficients are determined leading to an activation energy of 4.67 eV for Al diffusion in Cr2O3.