A method for operating a gas generation device, for example, for a fuel cell system, having at least two gas generation units through which a starting-material stream flows in series. The two gas generating units have a first and second rated power P.sub.rated.sub.--.sub.1, P.sub.rated.sub.--.sub.2 and a first and second predetermined operating temperature T.sub.rated.sub.--.sub.1, T.sub.rated.sub.--.sub.2, and the first gas generation unit has a lower thermal mass than the second gas generation unit. During a starting phase only the first gas generation unit is operated, with a power P.sub.start.sub.--.sub.1>P.sub.rated.sub.--.sub.1. After the end of the starting phase at least the second gas generation unit is operated.