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 ).