Oxygen permeation fluxes through dense SrFe0.67Co0.33O3-d membranes have been measured as a function of temperature, oxygen partial pressure gradient across the membrane, and membrane thickness. A maximum oxygen flux of 2.74 sccm/min cm(2) (1.87 x 10(-6) mol/s cm(2)) was measured at 1000 degreesC with air on the primary side, and He on the secondary side of the SrFe0.67Co0.33O3-d membrane. Activation energies from 88 to 108 kT/mol were found in the temperature range between 900 and 1000 degreesC. At 1000 degreesC with He on the secondary side of the membrane, a critical thickness of roughly 1.5 mm was found for SrFe0.67Co0.33O3-d where the rate limiting step for oxygen permeation changed from surface exchange kinetics to bulk diffusion. The critical thickness appears to be dependent on the temperature and the p(O2) gradient across the membrane. (C) 2000 Elsevier Science BN. All rights reserved.