Rapid response to a fuel cell system of the type including a reformer (32) in response to a change in load is achieved in a system that includes a fuel tank (24), a water tank (20) and a source (42) of a fluid at an elevated temperature. A heat exchanger (28) is provided for vaporizing fuel and water and delivering the resulting vapor to the system reformer (32) and includes an inlet (64) and an outlet (66) for the fluid. It includes a plurality of fluid flow paths (100), (102), (104) extending between the inlet (64) and outlet (66) as well as a fuel inlet (56) and a fuel outlet (58) spaced therefrom. The fuel inlet (56) and outlet (58) are connected by a plurality of fuel flow paths (52) that are in heat exchange relation with the fluid flow paths (100), (102), (104) and the fuel water inlet (56) is located adjacent the upstream ends of the fluid flow paths (100), (102), (104).