Abstract: Synthesis gases are prepared by partial autothermal oxidation of liquid fuels and/or solid fuels in the presence of oxygen or oxygen-containing gases with the addition of a temperature moderator, such as steam and/or CO.sub.2, in an empty reactor space without baffles, at from 1000.degree. to 1500.degree. C. under from 1 to 100 bar, the reactants fuel and oxygen-containing gas being fed separately to the reactor, by a process in which the steam or CO.sub.2 is fed in simultaneously with the feed of the fuel, and the steam is let down through one or more nozzles into the fuel steam directly before the orifice for the fuel, let-down being effected at from 30 to 250%, preferably from 80 to 140%, of the critical pressure ratio. An apparatus for carrying out the process is also described.