The effect of the bulk microstructure (grain size distribution, grain boundary length) on the oxygen transport properties of permeation membranes is investigated in this work. For this purpose, La0.5Sr0.5FeO3-? samples with different microstructures have been prepared by modifying the sintering duration and/or temperature. The average grain sizes, ranging from 0.20 to 1.43 micron, were determined from SEM analysis. The oxygen transport properties of these samples were characterised by permeation measurement as a function of temperature in an air/argon oxygen partial pressure gradient. The fluxes presented a change in the activation energy which was attributed to a change in the rate limiting step, from bulk diffusion at lower temperature (<850°C) to surface limitation at higher temperature (>900°C). Only the transport through the bulk was influenced by the microstructure, with the highest flux for the smallest grains. At 800°C, the fluxes were respectively 0.06, 0.03 and 0.01 micromol/cm2s through 1 mm thick samples of average grain sizes of 0.20, 0.63 and 1.43 micron, respectively. This would imply that oxygen transport occurs more rapidly along the grain boundaries than through the bulk. Grain boundary structure and composition were analysed by TEM.