A hydrogen generator (101) is provided which comprises a third chamber (109) containing a catalyst (121), a first chamber (103) containing a fluid, a second chamber (105) containing a material that reacts with the fluid in the presence of the catalyst to generate hydrogen gas, and a valve (111) movable from a first position in which the flow of fluid along a pathway including the first, second and third chambers is enabled, to a second position in which the flow of fluid along the pathway is prevented.