NOVELTY - The steam reformer comprises a first channel having a wall at least partly coated with a steam reforming catalyst, a second channel having a wall at least partly coated with a combustion catalyst, and a valve for selectively directing a first stream comprising air and fuel and a second stream comprising steam and fuel into the channels. One of the channels comprises a combustion channel and another comprises a steam reforming channel.    USE - The steam reformer is used for producing hydrogen to power a fuel cell for the production of electric power, e.g. for use in a vehicle.    ADVANTAGE - The reformer automatically regenerates itself by removing carbon deposits and can be operated indefinitely at peak efficiency. It ensures that the fuel will be fully converted in the steam reforming reaction. It is sufficiently compact to be used in a vehicle and minimizes the formation of sulfur deposits.    DETAILED DESCRIPTION - The steam reformer comprises a first channel having a wall which is at least partly coated with a steam reforming catalyst, a second channel having a wall which is at least partly coated with a combustion catalyst and positioned close to the first channel to permit heat transfer from the second to the first channel, and a valve for selectively directing a first stream comprising air and fuel and a second stream comprising steam and fuel into the channels. One of the channels comprises a combustion channel (3) and another comprises a steam reforming channel (2). An identity of the channels is determined according to a position of the valve. The steam reformer further comprises a measurement and control device connected to measure concentration of at least one component of an effluent from one of the channels. The valve is configured to reverse a direction of flow through the first and second channels. The reformer comprises mechanisms for introducing fuel into the second channel.    An INDEPENDENT CLAIM is also included for a method of making hydrogen, which comprises directing a first mixture of a hydrocarbon fuel and steam, and a second mixture of a hydrocarbon fuel and air into a valve having two positions, measuring a concentration of an effluent from at least one of the channels, and controlling the valve in accordance with a measured concentration of the effluent.    DESCRIPTION OF DRAWING(S) - The figure is a simplified schematic diagram of the steam reformer.    Steam reforming channel (2)    Combustion channel (3)    Middle strip (7)