Ethanol steam reforming in membrane reactors is a promising route for decentralized H-2 production from biomass because H-2 yield can be greatly enhanced due to the equilibrium shift triggered by instantaneous H-2 extraction. Here a highly active Ir/CeO2 catalyst has been combined with ca. 4 mu m thin Pd membranes employing a 6:1 steam/ethanol feed between 673 K and 873 K at reforming pressures up to 1.8 MPa. The H-2 yield reached 94.5% at 873 K and 1300 kPa due to the separation of 91.8% H-2 whereas H-2 yield was limited to 28.9% without membrane. At lower temperatures and pressures sweep gas was needed at the membranes' permeate side for efficient H-2 generation since the H-2 partial pressure remains equilibrium-limited on the reaction side. Furthermore, the H-2 yield improved from 63.0% to 84.7% at 773 K, 1500 kPa and sweep-to-feed flow ratio 0.5 when the distance between membrane and reactor wall was shortened by ca. 30%. Thus, external H-2 diffusion towards the membrane has a large impact on membrane reactor performance pointing towards microstructured membrane reactors as optimum devices for sustainable H-2 production from biomass. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.