An improved gas reformer for generating a gas mixture through the catalytic conversion of a vaporized, gasified or atomized liquid fuel which has been mixed with an oxygen containing gas, in which a plurality of successive reaction stages are provided so that fuel not converted in a first reaction stage is converted in a further, succeeding reaction stage. The oxygen containing gas is mixed with the output of each reaction stage which is to be converted in a further reaction stage and the amount of oxygen in the oxygen containing gas in the mixture provided to each reaction stage is controlled in that reaction stage such that the amount of oxygen in the gas mixture is increased with decreasing temperature.