We propose an oxygen surface exchange model in which the effect of vacancies at the gas-mixed ionic electronic conductor interface are included and apply the model to isotope exchange, oxygen permeability, and electrical conductivity relaxation. We deduce relationships between the surface-exchange coefficients associated with these phenomena and extend the treatment of the conductivity relaxation to large changes in oxygen partial pressure, where the commonly used assumption of first order reaction rate breaks down. We apply the model to interpret the permeation and electrical conductivity relaxation measurements in La 0.5 Sr 0.5 Fe 0.8 Ga 0.2 O 3Ϫ␦ . Transport in the material is almost completely surface limited, and data were interpreted in terms of a single surface-exchange coefficient.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.238.7.43 Downloaded on 2014-11-17 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.238.7.43 Downloaded on 2014-11-17 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.238.7.43 Downloaded on 2014-11-17 to IP