An increased awareness of potentially carcinogenic micropollutants in US drinking waters has led to a reevaluation of several well-established treatment techniques. The major focus of this concern has been on the recently recognized hazards of chlorination- the formation of both trihalomethanes and total organic halides. Coagulation has also been the subject of renewed investigation because of its ability to remove halogenated organic precursors. This article discusses the effects of preozonation and alum coagulation, both separately and in combination, on the removal of organic halide precursors.