The invention relates to an apparatus for furnishing a reaction mixture for a reforming catalyst of a fuel cell assembly, having a) an air infeed with means for regulating an air flow rate; b) a fuel infeed with means for regulating a fuel flow rate; c) a water vapor generator with means for regulating a water vapor flow rate; d) a liquid water infeed with means for regulating a liquid water flow rate; and e) a control unit, which communicates with the liquid water infeed, the water vapor generator, the fuel infeed, and the air infeed; and via the control unit, controlling variables for the fuel infeed, the liquid water infeed, the water vapor generator and the air infeed which are furnished as a function of a power demand to the fuel cell and as a function of an operating phase of the apparatus.