NOVELTY - Fuel is partially evaporated in a primary vaporization zone (1). The heat is supplied by thermal contact with a hot primary medium, superheated steam. This constitutes a first stage. The fuel is supplied with a second, oxidizing medium, air. Fuel which has already been evaporated, becomes partially-oxidized. The proportion of fuel which has not yet evaporated, does not take part in the reaction. Evaporation of this fuel is completed in a secondary vaporization zone (2), spaced away from the first. This constitutes a second stage.    USE - To vaporize e.g. diesel fuel for reforming and use in vehicle fuel cells.    ADVANTAGE - A complex liquid fuel, e.g. diesel, is completely vaporized without formation of soot, producing a feedstock suitable for reforming. Even high-boiling liquid fuel components are vaporized. The quality of vapor produced, is sufficient to ensure no soot formation during conversion in the reformer which follows. The mixing chamber described, carries out evaporation as described, and prepares the required oxidant-fuel mixture. The process is relevant to both diesel and petrol, the latter also containing a proportion of high-boilers. Reaction conditions and intermediates are discussed in the disclosure.    DETAILED DESCRIPTION - Ahead of the first stage, the fuel is finely-atomized. In the first stage, preferably 70-90% of the total fuel quantity is evaporated. The superheated steam is supplied at preferably less than 500degreesC. The air is supplied in counterflow to the fuel. Following partial oxidation in the first stage, The temperature of the reaction products is less than 800degreesC and preferably 450degreesC or less. The fuel is vaporized in a first stage and partially oxidized. It is completely mixed (6) with an oxidant (7) in a second zone, spaced away from the first. Before supplying the total oxygen requirement to the fuel, a fraction of preferably 35%-50%, is used for vaporization.    DESCRIPTION OF DRAWING(S) - The schematic view shows a longitudinal cross section of a suitable mixing chamber.    primary vaporization zone (1)    secondary vaporization zone (2)    secondary medium (5)    mixing zone (6)    oxidant (7)    fuel/oxidant mixture (8)    catalyst of autothermal reformer (9)