NOVELTY - A hydrogen generator is operated by determining the condition of the hydrogen generator and the condition of hydrocarbon-containing feed, electing predetermined feed rates for externally-provided raw materials based upon the determined conditions, and controlling the feed rate of each of the raw materials to the selected predetermined feed rates.    USE - For operating a hydrogen generator used to provide hydrogen to a fuel cell to generate electricity (claimed).    ADVANTAGE - The process can provide rapid transitions between hydrogen production rates while enabling enhanced efficiency and stability during transient operations.    DETAILED DESCRIPTION - Operating a hydrogen generator having a variable hydrogen production rate output, involves, in response to a hydrogen production rate demand, introducing externally-provided raw materials into the hydrogen generator to meet the hydrogen production rate demand, and generated hydrogen. The raw materials comprise hydrocarbon containing hydrocarbon-containing feed, oxygen-containing gas and water. Hydrogen is generated by reforming hydrocarbon-containing feed in the presence of steam to produce a reformate containing hydrogen, carbon monoxide and carbon dioxide; and converting carbon monoxide in the reformate to carbon dioxide by at least one of water gas shift and selective oxidation, and at least partially combusting hydrocarbon-containing feed with oxygen-containing gas to provide heat for reforming. The method includes determining the condition of the hydrogen generator and the condition of the hydrocarbon-containing feed, electing predetermined feed rates for the externally-provided raw materials based upon the determined conditions, and controlling the feed rate of each of the raw materials to the selected predetermined feed rates.    INDEPENDENT CLAIMS are also included for:    (a) process for changing the rate of hydrogen production by a hydrogen generator, comprising controlling the rate of change of the feed rate or each raw material in accordance with predetermined rate commensurate with the rate of change in the transition rate-limiting operation;    (b) process for transitioning during a transition period a hydrogen generator, comprising providing the ratio of raw material to fuel at a value different than such ratios at the steady state condition for the first hydrogen product rate and at the steady state condition for the second hydrogen product rate for the transition period to enhance the transition; and    (c) a method for maintaining low levels of carbon monoxide in a hydrogen fuel processor, comprising adjusting a water to hydrocarbon fuel ratio and an air to hydrocarbon fuel ratio in accordance with a predetermined algorithm, reducing concentration of the carbon monoxide in the hydrogen fuel by passing the hydrogen fuel first to water gas shift reactor(s) and then to preferential oxidation reactor(s), where the algorithm comprises determining a target hydrocarbon fuel flow (B) and a current hydrocarbon fuel flow (A), then determining a present difference (D) = (B)-(A), and then 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, and where 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 of time than when the fuel processor is operating at a steady state mode and where a lower ratio of water to fuel and air to fuel is added when the fuel processor is in a turning down of production mode.    DESCRIPTION OF DRAWING(S) - The figure shows the responses of three conditions in a hydrogen generator to a change from hydrogen production rate A to rate B.    = slow transient response (C1)    More rapid catalyst temperature (C2)    Even more rapid fuel to oxygen-containing gas ratio in the feed to the reformer (C3)