This work investigates the performance of iron cobalt oxide silica membranes for the separation of binary gas mixtures containing H-2 and Ar up to 500 degrees C. A series of membranes were prepared by fixing the iron/cobalt molar ratio at 10/90, 25/75 and 50/50. H-2 preferentially permeated though the membranes, and H-2 purity in the permeate stream increased with temperature for all H-2/Ar binary gas mixtures. The fluxes of H-2 from binary gas mixtures complied, for the most part, with a temperature dependent transport mechanism, similar to that delivered by single gas permeation. However, it was notable to observe a "transition point" where H-2 purity versus H-2 flux clearly changed from temperature independent to temperature dependent. This gas separation transition point was also found to be a function of the quality of the membrane. Indeed the best performing membrane (Fe/Co = 10/90) also had the highest gas separation transition point at similar to 70% H-2 purity. This reduced to similar to 60% for the medium quality membrane (Fe/Co = 25/75) and was at its lowest similar to 43% for the lower quality membrane (Fe/Co = 50/50). The binary gas fractions therefore affect the H-2 fluxes and H-2 purity more significantly than that expected in single gas permeation. Therefore, the relationship between membrane quality and gas separation transition point is established for the first time in this work. (C) 2015 Elsevier B.V. All rights reserved.