In this study, a mathematical model has been developed for permeation of ions through a thin mixed conducting solid oxide membrane. This model correlates transport properties of mobile defects to some important parameters such as diffusion coefficient, film thickness. temperature and gas partial pressures. The theoretical analysis was carried out using the SrCe0.95Yb0.05O2.975 (SCY) mixed conducting membrane and the effects of various parameters on the ionic permeation are discussed in detail. The predicted values generated from the present model show good agreement with literature experimental data given by Hamakawa et al. [J. Electrochem. Sec. 141 (1994) 1720]. Based on the available equilibrium data of the (SCY) mixed conducting membrane, the simulated results reveal that. at an operating temperature of less than 1200 K, the membrane behaves as a mixed proton-hole conductor, while it switches to a mixed oxygen vacancy-hole conductor when the operating temperature is increased to 1600 K. (C) 2000 Elsevier Science B.V. All rights reserved.