NOVELTY - A fuel processor (1) for converting a hydrocarbonaceous fuel into a product gas, preferably containing hydrogen and carbon dioxide, comprises at least one catalytic zone (5B) and a chamber (11) for storing oxygen, which communicates with the catalytic zone, or a fluid supply, which communicates with the catalytic zone and which is connected or connectable to an oxygen source.    USE - The fuel processor is used for converting a hydrocarbonaceous fuel into a product gas, preferably containing hydrogen and carbon dioxide (claimed), useful for generating heat and power for fuel-cell-powered vehicles and domestic systems.    ADVANTAGE - The catalytic zone can be heated effectively by exothermic reactions of a fuel or an intermediate product and oxygen taking place on the catalyst in the catalytic zone itself. The fuel processor can be started up faster. Parts of the catalytic zone that reach working temperature first, will also contribute to reducing start up time because the water-gas shift reaction itself is exothermic and heat generated in those parts through this reaction will accelerate heating of the parts that have not yet reached the working temperature. During start up and before feeding a mixture of a fuel and oxygen to the fuel processor, an oxygen rich gas, such as air of exhaust gases from a burner, is fed to the fuel processor thus filling the chamber with the gas. Filling of the chamber followed by controlled diffusion of the oxygen can be improved by feeding the oxygen rich gas to the fuel processor at a lower mass flow rate than the mixture.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method of starting up a fuel processor for converting a hydrocarbonaceous fuel into a product gas, comprising heating of part of the catalytic zone to a working temperature by exothermic reactions taking place on the catalyst in the catalytic zone.    DESCRIPTION OF DRAWING(S) - The figure shows the section between the water-gas shift reaction zones in more detail.    Fuel processor (1)    Catalytic zone (5B)    Chamber (11)    Inlet portion (12)    Nozzle (15)    Conduit (16)    Opening (17)