A process and device for determination of the lambda value (.lamda..sub.actual) of reformate (10) which is to be supplied to a fuel cell stack (12), by determining the lambda value (.lamda..sub.actual) of the no-load voltage (U.sub.0) of the voltage on at least one fuel cell element (14). Furthermore, a process and device for lambda control of a reformer, utilizes the process and device for determining the lambda value to controller an actuator of a lambda-controlled reformer (16) for reaction of at least fuel (20) and air (22) into reformate (10), the reformer being connected to a fuel cell stack (12) to supply reformate (10) form the reformer (16) to the fuel stack (12).