A fast start-up catalytic reformer for producing hydrogen-rich reformate from hydrocarbon fuel includes a reactor having an inlet for receiving a flow of fuel and a flow of air, a reforming catalyst disposed within a reforming chamber in the reactor, and an outlet for discharging the produced reformate stream. An ignition device within the reactor tube ignites a first lean mixture in combustion mode to generate exhaust gases to warm the catalyst which also warms the wall of the reactor adjacent the catalyst. The reactor then switches over to a rich fuel/air mixture during reforming mode. A jacket concentrically surrounds the reactor, defining a mixing chamber therebetween which communicates with the reforming chamber via openings in the wall of the reactor. Fuel entering the reformer in combustion mode is injected directly into the reforming chamber to provide rapid warming of the catalyst. Fuel entering the reformer in reforming mode is sprayed onto the outside of the reactor in the mixing chamber, preferably in the heated region of the tube for very rapid vaporization.