A novel injector/reactor apparatus and an efficient process for the partial oxidation of light hydrocarbon gases, such as methane, to convert such gases to useful synthesis gas for subsequent hydrocarbon synthesis. Sources of a light hydrocarbon gas, such as methane, and oxygen or an oxygen-containing gas are preheated and pressurized and injected through an injector means at high velocity into admixture with each other in the desired proportions, preferably at comparable momentums, at a plurality of mixing nozzles which are open to the reaction zone of a reactor and are spaced over the face of the injector, to form a reactant gas premix having a pressure drop equal to at least 1% of the lowest upstream pressure of either of said gas streams. The gas premix is ejected in a time period which preferably is less than 9 milliseconds, at a velocity between about 25 to 1000 feet/second, into a reaction zone comprising a partial oxidation zone, so that the gas mixture reacts therein. The partially-oxidized effluent is then passed into a steam reforming catalyst zone to reduce the amounts of CO.sub.2, H.sub.2 O and heat produced by the partial oxidation reaction to form the useful syngas, which is cooled, recovered and/or further processed.