Raw wastewater, secondary effluent, lime clarified and filtered raw and secondary wastewaters, carbon treated wastewaters, and breakpoint chlorinated plus carbon treated wastewaters were ozonated at pH 7.0 over a range of 5-90 minutes contact time. Ozonation of the raw wastewater, with high solids and COD (Chemical Oxygen Demand) content required impractical ozone dosages for appreciable COD removal. In all effluents, except raw wastewater, 100 mg/l of ozone produced at least a 70% COD removal. Organic oxidation efficiencies in the raw and secondary wastewaters, based upon one atom of available oxygen per molecule of ozone, exceeded 100% indicating that one or more atoms of the ozone molecule or molecular oxygen participated in the organic oxidation mechanism. Variable amounts of organic nitrogen and ammonia were oxidized at pH 7.0 by ozone to nitrate. The ozone distribution ratio between oxidation of the NOD and COD, and the COD removals as a function of ozone dose suggested that the relative order of oxidation was easily oxidizable organic material, NH3, TKN, and slowly oxidizable (nearly refractory) organic material. (Modified author abstract)