A method and apparatus are provided for regulating electrical power output of a fuel cell system comprising a fuel processing system, a system for supplying an oxidant stream, and a fuel cell to supply power. In a reformer of the fuel processing system a supply fuel is used to produce hydrogen-rich gas to be supplied to the fuel cell. The fuel cell system also contains a controller to set the mass flow of the supply fuel to the fuel processing system and the mass flow of the oxidant stream to the fuel cell, whereby the mass flow of the oxidant stream can be set in dependence on the dynamic response of the fuel processing system and/or the mass flow of the hydrogen-rich gas can be set in dependence on the dynamic response of the system for supplying the oxidant stream.