An autothermal membrane reformer comprising two separated compartments, a methane oxidation catalytic bed and a methane steam reforming bed, which hosts hydrogen separation membranes, is optimized for hydrogen production by steam reforming of methane to power a polymer electrolyte membrane fuel cell (PEMFC) stack. Capitalizing on recent experimental demonstrations of hydrogen production in such a reactor, we develop here an appropriate model, validate it with experimental data and then use it for the hydrogen generation optimization in terms of the reformer efficiency and power output. The optimized reformer, with adequate hydrogen separation area, optimized exothermic-to-endothermic feed ratio and reduced heat losses, is shown to be capable to fuel kW-range PEMFC stacks, with a methane-to-hydrogen conversion efficiency of up to 0.8. This is expected to provide an overall methane-to-electric power efficiency of a combined reformer-fuel cell unit of similar to 0.5. Recycling of steam reforming effluent to the oxidation bed for combustion of unreacted and unseparated compounds is expected to provide an additional efficiency gain. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 525-541, 2011