A novel fluidized bed syngas (FBSG) injector/reactor apparatus and an efficient process for the partial oxidation and steam reforming of light hydrocarbon gases such as methane, to convert such gases to useful synthesis gas containing CO and H.sub.2 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 gas orifices of an injector at high velocity and comparable momentums into admixture with each other in the desired proportions, at a plurality of mixing chambers or recessed cups which are open to the fluidized bed reaction zone of a reaction chamber and are spaced over the face of the injector, to form a reactant gas premix having a pressure drop of at least 1% through the injector. 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 partial oxidation reaction zone comprising a fluid bed catalyst so that the gas mixture reacts in the catalyst bed, to reduce the amounts of CO.sub.2, H.sub.2 O and heat produced by the partial oxidation reaction to favor the desired stoichiometry. The formed syngas is cooled and recovered, such as use in further synthesis processing.