A vaporizer, a fuel cell system including the vaporizer, and a method of vaporizing fuel in a fuel cell system are disclosed. The fuel cell system includes a fuel reservoir (24) for storing a liquid fuel and a fuel cell (10) for consuming a fuel and generating electricity therefrom. A fuel vaporizer (28) is interposed between the fuel reservoir (24) and the fuel cell (10) for receiving liquid fuel and vaporizing it and delivering it ultimately to the fuel cell (10). The fuel vaporizer (28) includes a heat exchanger which includes a hot fluid inlet (65), a hot fluid outlet (67) and a core (50) interconnecting the inlet (65) and the outlet (68). The core (50) has alternating fuel flow structures (68) and hot fluid structures (69) with the fuel flow structures (68,69) having an inlet (56) and an outlet (58).