The invention relates to a process for determining the lambda value (λactual) of reformate provided for supply to a fuel cell stack, in which the no-load voltage across at least one fuel cell element is detected and evaluated for determining the lambda value (λactual). In accordance with the invention it is provided for that the at least one fuel cell element is a terminal fuel cell element of the fuel cell stack exclusively provided for sensing, and the voltage provided for at least one consumer can be picked off across the remaining fuel cell elements of the fuel cell stack. The invention relates furthermore to a process for lambda control of a reformer, a device for determining the lambda value, as well as to a system comprising a reformer for reacting at least fuel and air into reformate, and a fuel cell stack which is connected with the reformer for receiving reformate therefrom, the reformer being lambda-controlled.