Based on a new isothermal approach a reactor design for the autothermal reforming of methanol is developed. The new reactor concept addresses the special requirements of mobile applications, such as compact design and good dynamic behaviour. With the catalyst carried in porous disks, an even temperature profile combined with a constant flow distribution can be achieved. The disks provide a copper-matrix, that fixes the small catalyst-particles, transports heat and provides an open structure for mass-transport. Therefore heat and mass transfer are not limiting, and the reactor is operated at optimum temperature, resulting in high reaction rates with low concentrations of by-products like carbon monoxide or methane. There are no changing temperature profiles and negligible product-fluctuations as a response to load steps of more than one order of magnitude. This enables the easy and precise control of the reaction. For start-up the reactor is heated by catalytic oxidation of methanol. Starting from room-temperature, a temperature of 300 degreesC is reached within less than a minute.