Abstract: A method for treating waste water streams by introducing a mixture of pre-heated waste water and an amount of oxidant lower than that required to oxidize all the organic matter in the waste stream into the front port of an elongate tubular reaction chamber. The amount of oxidant is preferably high enough, however, to raise the temperature of the stream in the reaction chamber to 750.degree.-1,200.degree. F. A first injection side port is preferably located downstream from the front port of the reaction chamber at a distance at which the temperature has reached a maximum value, and 90-100% of the available oxidant has been consumed. Oxidant and water may be simultaneously injected through the first port, in a manner that the amount of oxidant is adequate to oxidize a specified amount of organic matter, and the amount of water is effective to counterbalance the heat produced by the reaction of the co-injected oxidant with the respective organic matter.