A dynamic model of a polymer electrolyte fuel cell hybrid system has been developed within MATLAB -SIMULINK . Components are modeled using electrochemical and mass transport as well as heat transfer equations. The implemented equations describe the steady-state as well as the dynamic operation of the system with sufficient accuracy, although considerable simplifications have been made for the stack and the peripheral components to keep model complexity and computing time low. Emphasis is given to the operation limits of the PEFC system, notably the conditions for trouble-free operation at different loads and high or low ambient temperature. The potential of the simulation as system optimization tool and efficient operation guide are demonstrated. Model validation was accomplished by experiments on a homemade portable system including a accumulator for peak power output.