Abstract: NO.sub.x reduction in a combustion product flue gas stream is greatly improved by the efficient mixing of the reducing agent, e.g., NH.sub.3 or an NH.sub.3 precursor, with the stream in the absence of a catalyst, such as CaO or CaCO.sub.3, that promotes undesirable combination of NH.sub.3 and O.sub.2 to form additional NO.sub.x. By employing a high-temperature cyclone separator of the like and by injecting the reducing agent into the stream within such hot cyclone at a location where there is a strong vortex region, extremely efficient mixing of the NO.sub.x reducing agent and the combustion product flue gas is promoted after particulates which are harmful to the reaction that promotes NH.sub.3 and NO.sub.x reduction have been separated from the flue gases. A refractory-lined cyclone separator may be employed, and a plurality of injectors used which discharge liquid or gas into the strong vortex region.