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 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 relative proportions, at a plurality of mixing nozzles which are open to the catalytic partial oxidation reaction zone and are uniformly-spaced over the face of the injector means, to form a reactant gaseous premix having a pressure drop equal to at least about 3% of the lowest upstream pressure of either of said 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 a catalytic partial oxidation zone so that the gaseous premix reacts in the presence of the fixed catalyst to reduce the amounts of CO2, H2O and heat produced by the partial oxidation reaction, to form a useful syngas which is cooled and recovered.