A point defect model was used to describe the oxygen nonstoichiometry of the perovskites La0.75Sr0.25CrO3, La0.9Sr0.1FeO3, La0.9Sr0.1CoO3 and La0.8Sr0.2MnO3 as a function of the oxygen partial pressure. Form the oxygen vacancy concentration predicte by the point defect model, the ionic conductivity was calculated assuming a vacancy diffusion mechanism. The ionic conductivity was combined with the Wagner model for the oxidation of metals to yield an analytical expression for the oxygen permeation current density as a function of the oxygen partial pressure gradient. A linear boundary condition was used to show the effect of a limiting oxygen exchange rate at the surface
