Fuel cell powered vehicles with on board reforming need compact and lightweight components. A membrane reactor, that combines hydrogen permeable membranes with a methanol steam reformer promises considerable weight and space savings. Its dense metal membranes produce high purity hydrogen over a wide range of pressure and load. The selective removal of hydrogen yields methanol- and CO-conversions that are higher than the equilibrium conversion in a conventional reactor. Results with three different metal membranes in a membrane reactor for steam reforming of methanol are presented. A mathematical model accurately describes the measured performance of the membrane reactor and allows predictions for other values of the process parameters.