NOVELTY - Maintaining low levels of carbon monoxide in a hydrogen fuel processor involves adjusting the water to hydrocarbon fuel ratio and the air to hydrocarbon fuel ratio according to a predetermined algorithm. The algorithm comprises determining the present difference between the target hydrocarbon fuel flow and the current hydrocarbon fuel flow, and comparing the difference with a predetermined threshold value.    USE - Maintaining low levels of carbon monoxide in a hydrogen fuel processor.    ADVANTAGE - The process solves the problem of maintaining a low carbon monoxide concentration during transient operation, especially during turn-up.    DETAILED DESCRIPTION - Maintaining low levels of carbon monoxide in a hydrogen fuel processor involves adjusting a water to hydrocarbon fuel ratio and an air to hydrocarbon fuel ratio in accordance with a predetermined algorithm, where the fuel processor comprises a supply of the hydrocarbon fuel, and water and steam supplied to a reactor to produce hydrogen fuel comprising hydrogen and carbon monoxide, followed by reduction in concentration of the carbon monoxide in the hydrogen fuel by passing the hydrogen fuel to at least one water gas shift reactor (9) and to at least one preferential oxidation reactor (12, 14).    The water is added to the hydrocarbon fuel prior to the hydrocarbon fuel entering the reactor. Air is added to the preferential oxidation reactor in accordance with the algorithm. The algorithm comprises determining a target hydrocarbon fuel flow (B) and a current hydrocarbon fuel flow (A), determining a present difference (D) = (B) - (A), and comparing the difference (D) with a predetermined threshold value to determine whether the fuel processor is turning up production of hydrogen, turning down production of hydrogen, or operating at a steady state mode. A higher ratio of water to fuel and air to fuel is added when the fuel processor is turning up production for a preset period than when the fuel processor is operating at a steady state mode. A lower ratio of water to fuel and air to fuel is added when the fuel processor is in a turning down production mode.    DESCRIPTION OF DRAWING(S) - The figure is a schematic block flow diagram illustrating the core process.    Preheat exchanger (4)    Autothermal reforming reactor (7)    Water gas shift reactor (9)    Preferential oxidation reactor (12, 14)