Oxygen flux through dense membranes with composition SrFeO3-deltaSrFe0.67Co0.33O3-delta and Sr0.97Fe0.33Co0.67O3-delta (0, 33% and 67% Cc) have been measured primarily at 1000 degreesC as a function of membrane thickness with synthetic air on the primary side and He on the secondary side. Maximum flux rate was found to increase with cobalt content with an observed maximum for a membrane with composition 67% cobalt and thickness 0.5 turn corresponding to 3.8 X 10(-6) mol/cm(2) s. Based on experimental data, the critical thickness has been estimated to decrease from 2.0 mm for SrFeO3-delta to 0.7 mm for a compound with 67% cobalt added. A severe oxygen flux reduction is observed below approx. 900 degreesC for membrane samples with composition Sr0.97Fe0.33Co0.67O3-delta, corresponding to a transition from a cubic perovskite with disordered oxygen vacancies to a rhombohedral brownmillerite structure with ordered oxygen vacancies.