Abstract We report several structural and chemical phenomena that were observed after high temperature annealing (1200 °C) of reduced 5 at.% Nb-doped polycrystalline SrTiO 3 in oxygen. The oxidation led to the formation of islands on the surface which, depending on the surface orientation of the grains, exhibit characteristic shapes, densities, and lateral directions. A correlation between grain orientation and tendency towards island formation was found. Structure and composition of the islands and the bulk were investigated by transmission electron microscopy. The bulk interior of the oxidized SrTiO 3 was found to exhibit compositional changes and Sr-rich secondary phases were found at the triple grain junctions. The results are discussed in terms of the defect chemistry of SrTiO 3.