In this paper we present results of our theoretical studies targeted at anion mobility in solid M-Ta-O-N systems. Periodic supercell calculations at density-functional level have been performed to investigate the local structures of N-doped tantalum oxides and their anion diffusion mechanisms. The migration pathways and activation barriers were calculated using the nudged elastic band method with the climbing-image enhancement. We show that the defect migration is mainly caused by the diffusion of oxygen anions. The activation energy can be lowered by increasing the defect concentration, and it is, to a large extent, depending on the dopant size. (C) 2007 Elsevier B.V. All rights reserved.