Oxygen permeation measurements were performed on dense La1−xSrxFeO3−δ (x = 0.1-0.4) membranes in large oxygen activity gradients, i.e. air/CO, CO2, in the temperature range 1173–1323 K, yielding fluxes up to 25 mmol m−2 s−1 at 1273 K. The oxygen semipermeability was linearly proportional with the CO partial pressure and the strontium content. It was concluded that the fluxes are limited by the surface exchange kinetics. Two simple models are proposed for the oxygen exchange reaction in the presence of CO. In both models oxygen vacancies at the phase boundary play a definite role in the exchange process. XPS and SEM analysis of the perovskite/CO, CO2 interfaces indicated segregation of strontium, being present at the surface mostly in the form of SrCO3 and/or SrO. The enhanced oxygen fluxes in air/He gradients as they were observed after exposure of the membrane surface to CO are probably (indirectly) related to the level of strontium segregation. Based on SEM analysis and surface profile measurements, it is suggested that the segregation process is accompanied by an enlargement of the specific surface area, which may promote the speed of the overall permeation process under exchange-controlled conditions.