Oxygen permeation measurements in air/He gradients were performed on dense La1 ¿ xSrxFeO3 ¿ ¿ membranes in the composition range x = 0.1¿0.4 and temperature range 1123¿1323 K. Pretreatment of the lower oxygen partial pressure side of the membranes in a CO-containing atmosphere for several hours at 1273 K led to higher oxygen fluxes, which were in the range of 0.1¿4.5 mmol m¿2 s¿1. After treatment, the observed oxygen fluxes could be described in terms of bulk diffusion-limited permeation behaviour. Experimental evidence for a bulk-diffusion controlled flux was found from thickness dependence measurements on membranes with thicknesses between 0.5 mm and 2.0 mm. Model calculations, based on Wagner theory in conjunction with data of oxygen nonstoichiometry and vacancy diffusion coefficients from literature, were performed. The experimental flux values deviated from the model calculations with factors up to 2.5. Adjustment of the value of the vacancy diffusion coefficient led to good agreement between the experimental data and the model calculations. The calculated vacancy diffusion coefficients Dv0 were virtually independent of composition and were found to be in the range 5.3¿9.3 × 10¿6 cm2 s¿1.