Thin (similar to 30 mu m), dense Ce0.9Gd0.1O1.95-delta (CGO10) membranes (5 x 5 cm(2)) supported on a porous NiO/YSZ substrate were fabricated by tape casting, wet powder spraying and lamination. A La0.58Sr0.4Co0.2Fe0.8O3-delta/Ce0.9Gd0.1O1.95-delta (LSCF/CGO10) composite cathode was applied by screen printing. Oxygen permeation measurements and electrochemical characterisation of the cells were performed as a function of temperature with air and varying hydrogen/steam mixtures flowing in the feed and permeate compartments, respectively. The oxygen flux was found to reach 10 N mL min(-1) cm(-2) at similar to 1100 K and to exceed 16 N mL min(-1) cm(-2) at 1175 K. The measured oxygen flux was in good agreement with theoretical predictions from a model that takes into account the bulk transport properties of Ce0.9Gd0.1O1.95-delta, the anode and cathode polarisation resistances, and the gas conversion and gas diffusion losses in the permeate compartment. The performance of the membrane was also investigated under varying CH4 and H2O gas mixtures at 1106 K. The oxygen flux increased with decreasing steam to carbon ratio and was found to exceed 10 N mL min(-1) cm(-2) of O-2 for steam to carbon ratios below 4:3. Post-test analysis of the tested membrane did not reveal any significant microstructural degradation of the CGO10 membrane or the anode-support. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3559189] All rights reserved.