The effect of the bulk microstructure (grain size distribution, grain boundary composition on the oxygen transport properties of La_0.5Sr_0.5FeO₃membranes was investigated. For this purpose, samples with different microstructures were prepared by modifying the sintering duration and/or temperature. The average grain sizes, ranging from 0.20 to 1.43μ m, were determined from SEM analysis. The oxygen transport properties of the samples were characterised by permeation measurement. 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 limitations at higher temperature (>900°). Only the transport through the bulk was influenced by the microstructure, with the highest flux for the smallest grains. This would imply that oxygen transport occurs more rapidly along the grain boundaries that through the bulk. Grain and grain boundary compositions were analysed by TEM.