A method of operating a gas generating system for generating a hydrogen-rich product gas by partial oxidation or autothermal reforming includes operating a reactor approximately adiabatically. The ratio of the mole flow O.sub.2/mole flow C-atoms.sub.fuel (O.sub.2/C ratio) is controlled on the basis of the temperature of the educts and of the reaction temperature. For adjusting and maintaining an optimal O.sub.2/C ratio with respect to a high hydrogen yield, approximately an O.sub.2/C ratio corresponding of the present educt temperatures is adjusted and is corrected on the basis of the deviation of the reaction temperature from a desired reaction temperature. The O.sub.2/C ratio is then adjusted by controlling the oxygen-containing educt or the hydrocarbon-containing educt.