Oxide-ion conductors are used in sensors, oxygen pumps and solid oxide fuel cells. However, their range of application is limited by a low oxide-ion conductivity in the solid electrolyte presently commercially available. The electrical conductivity and the oxygen exchange behaviour of polycrystalline ceria-lanthania specimens with different grain boundary areas have been studied in an attempt to improve the grain boundary conductivity. The results show that a large increase in the grain boundary conductivity was obtained. The thermodynamic properties of the oxygen nonstoichiometric defect reaction of Ce1-nLanO2-0.5n-x solid solutions were measured by coulometric oxygen titration.