Composite membranes comprised of vanadium foils coated with molybdenum carbide catalyst layers were recently introduced as alternatives to palladium for high temperature separation of H-2. Experiments using D-2/H-2 mixtures unambiguously show that the mechanism involves dissociation, proton transport, and subsequent recombination. Temperature-dependent measurements of H-2 flux were performed on sets of membranes in which the thickness of both the V foil and the MO2C layers were varied in order to provide insight into the underlying transport mechanisms. It is shown that hydrogen transport through the carbide itself can be limiting for catalyst layers > 20 nm. At temperatures <750 degrees C the flux is highly activated, suggesting that dissociation of H-2 on the carbide surface is the rate limiting step. At higher temperature the permeability decreases with temperature in good agreement with the theoretical predictions of the permeability of the underlying V metal. The permeability of these composite membranes significantly exceeds that of pure Pd, with values as high as 5.9 x 10(-8) mol m/m(2) s Pa-0.5 at 750 degrees C. (C) 2012 Elsevier B.V. All rights reserved.