An apparatus is provided which comprises a thermosiphon shift reactor for the conversion of carbon monoxide and the generation of steam for use in a fuel processor for the conversion of hydrocarbon fuels into hydrogen and employs the hydrogen in a fuel cell. The thermosiphon shift reactor comprises a vertically extended shell defining an interior volume containing a vertical axis defining a catalyst zone. The vertically extended shell contains a catalyst zone comprising a shift catalyst wherein a high temperature shift catalyst is disposed in successively tapering layers relative to a low temperature shift catalyst and the catalyst zone is at least partially surrounded by an external jacket which defines a flow passage between the vertically extended shell and the external jacket. A fluid reservoir is located above the vertically extended shell at a height effective to establish a thermosiphon effect between the flow passage and the reservoir which generates steam and provides an essentially uniform temperature at the shell wall.