The oxygen permeation through an oxide ceramic membrane with bulk composition SrFeCo0.5O3.25-delta has been measured as a function of both temperature and the oxygen pressure gradient across the membrane. The form of the pressure gradient dependence of the permeation indicates that the oxygen transport in the membrane is dependent primarily on the bulk diffusion rate. Although the permeation experiments were carried out at temperatures within, or very close to, the range where SrFeCo0.5O3.25-delta is stable as a pure single phase in air, the membrane was found to consist of SrFe1.5-xCoxO3.25-delta (x = similar to 0.42) together with fractions of Sr(Co1-xFex)O2.5-delta perovskite and Co-Fe oxide. Phase stability experiments indicate these phases formed as a stable assemblage when the membrane was densified at high temperature (1090 degrees C) during final processing. The assemblage persists at the lower temperatures of the permeation experiments most likely because of slow re-equilibration kinetics.