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 recovery and/or subsequent hydrocarbon synthesis. Sources comprising a light hydrocarbon gas, such as methane, and oxygen or an oxygen-containing gas, preheated and pressurized, are injected through an injector means at high velocity into admixture with each other in the desired relative proportions, at a plurality of mixing nozzles which are open to the partial oxidation zone of a reactor and are uniformly-spaced over the face of the injector means, to form a reactant gaseous premix having a pressure at least 3% lower than the lowest upstream pressure of either of the streams of the individual gases. The gaseous premix is injected in a time period which is less than its autoignition time, preferably less than 9 milliseconds, at a velocity between about 25 to 1000 feet/second, into the partial oxidation zone of the reactor. The gas mixture reacts before or simultaneously with the autoignition time delay of the mixture, to reduce the amounts of CO.sub.2, H.sub.2 O and heat produced by the partial oxidation reaction to form a useful syngas which is cooled and recovered.