A three-dimensional finite element method (FEM) model that enables the performance simulation of mixed ionic-electronic conducting (MIEC) oxygen transport membranes (OTM) has been developed. In order to evaluate the influence of a porous functional layer on the membrane performance a numerical geometry generator was implemented that allows to create arbitrary porous microstructures. The 3D OTM model includes the spatially coupled physicochemical processes i) gas diffusion in the porous functional layer, ii) oxygen exchange at the feed-side between gas phase and MIEC material, iii) oxygen ion diffusion across the membrane, iv) oxygen excorporation at the permeate-side. The performed simulation carried out for the state-of-the-art MIEC composition La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) was validated with the help of oxygen permeation measurements carried out on an asymmetric LSCF thin-film OTM in the temperature range of 750...1000 degrees C. The simulation results identified a surface exchange dominated regime for membrane thicknesses below 50 mu m. While the application of a porous functional layer on the feed side could only increase the permeation flux by around 26%, the model demonstrates the significant improvement by a factor of two (for the given conditions) that can be achieved with a functional layer on the permeate side in case of a 20 mu m thin-film membrane. (C) The Author(s) 2014. Published by ECS. All rights reserved.