Gadolinium doped ceria (Ce(0.1) Gd(0.9)O(1.95-delta), CGO10) was investigated as oxygen separation membrane material for application in syngas production. Planar, thin film CGO10 membranes were fabricated by tape casting and lamination on porous NiO-YSZ supports and subsequent co-sintering. High oxygen fluxes, up to 16 N ml cm(-2) min(-1) at 900 degrees C, were obtained when placing the membrane between air and humidified hydrogen (H(2)/H(2)O= 20). Initial experiments for syngas production were performed by testing the CGO10 membrane with methane and steam feed. The mechanical integrity of CGO10 membranes during operation (heat up, cooling, reduction and re-oxidation) was also investigated. Chemically induced stress in the CGO10 membrane due to harsh reduction of the CGO10 material at high temperatures and very low pO(2) values can lead to mechanical failure by lattice expansion. Calculations of the oxygen non-stoichiometry profile in the 30 mu m thin CGO membrane under operation reveal that due to oxygen permeation in the membrane the largest non-stoichiometry at the permeate (fuel) side is more than a factor of 6 times smaller at 850 degrees C than that expected for CGO10 at equilibrium. The related relative expansion of the thin film CGO membrane should therefore lie below the expansion limit of 0.1% expected to be critical for mechanical stability and thereby allows for operation at high temperatures and low oxygen partial pressures. (C) 2010 Elsevier B.V. All rights reserved.