A system and method for controlling the bleed valve in an anode recirculation loop of a fuel cell system. The system uses a model to determine the concentration of hydrogen in the recirculated gas by calculating the volume flow of the recirculated gas through a fuel cell stack, a pressure drop across the anode inlet and outlet of the stack, and the density of the recirculated gas, and using a measured temperature of the recirculated gas and a measured pressure drop across a recirculation pump.