The reaction and interdiffusion characteristics at the interface of rare-earth doped ceria (M0.2Ce0.8O1.9, M = Gd, La) and perovskite-type cathode materials (La0.8Sr0.2CoO3, and La0.8Sr0.2FeO3) were investigated and their effect on the long-term reliability of solid oxide fuel cell (SOFC) performance are discussed. Secondary phase formation was observed at the ceria side of the interface by the reaction of the rare-earth component in ceria and transition metal components in perovskite. A significant diffusion of transition metals (cobalt or iron) and strontium from perovskite to ceria was observed. The iron content in perovskite and the type of rare earth in ceria strongly affect the stability of secondary phase and extent of transition metal diffusion in ceria. The bulk diffusivity was determined as a function of temperature. The iron component exhibited a fast and inhomogeneous migration in ceria, forming the iron-rich inclusions. The strontium component exhibited the tendency of the fast grain boundary diffusion in ceria. The effects of mass transport on the SOFC materials durability are discussed. (c) 2007 The Electrochemical Society.