The transfer of oxygen across an oxygen transport membrane (OTM) is limited by a number of processes, such as mass transfer across a boundary layer on the airside, surface exchange, ambipolar diffusion through the mixed-conducting gas separation layer, and viscous flow of oxygen through the porous support. Each of these processes was incorporated into a comprehensive oxygen transfer model. This model describes oxygen flux as a function of temperature, pressure, and oxygen recovery across composite OTMs. The model predicts an optimum thickness for porous mixed-conducting layers to improve oxygen surface exchange. Layer thickness depends on a number of parameters, such as pore size distribution, porosity, and tortuosity in each layer, and basic material parameters, such as surface exchange rate and ambipolar conductivity. The transfer model shows the importance of optimizing these parameters and a reactor design that enables a high mass transfer coefficient on the airside of the OTM element for optimum performance. (C) 2004 Elsevier B.V. All rights reserved.