Fabrication of dense perovskite membranes in the form of capillaries or hollow fibres is considered attractive for large-scale oxygen separation applications. For the preparation of such membranes by phase-inversion process polysulphone or polyethersulphone are commonly used as a binder. The decomposition of the sulphur-containing binder during the calcination leads to the formation of sulphates, which negatively affect the oxygen permeation through the membrane. The present work focuses on the comparative analysis of the oxygen transport mechanism through sulphur-free and containing Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) membranes. The analysis of the thickness dependence of the oxygen permeation fluxes indicated that sulphates decrease the permeation rate mostly due to the partial blocking of the surface oxygen exchange, whilst the bulk ambipolar conductivity remains essentially unchanged. SEM/EDS studies revealed segregation of BaSO4 at the grain boundaries, which might be responsible for the fast oxygen exchange in phase-pure BSCF. The negative impact of sulphur contamination on oxygen permeation was more pronounced at temperatures below 1123 K. It has been demonstrated, that, by surface activation, the oxygen flux through sulphur-containing BSCF membranes can be increased to the level of that of sulphur-free membranes. (C) 2012 Elsevier B.V. All rights reserved.