This study investigated the effects of ozone on several water quality parameters that affect particle behavior in water treatment systems. Among the most surprising results were those indicating what ozone did not do. Ozone did not decrease critical coagulant concentration for AlCl3, FeCl3, or alum coagulants; destabilize particles; or improve particle removal processes if pH was held constant. There was no evidence that disruption or desorption of the organic coatings of particles occurred or, if it did, that it enhanced particle destabilization. In sum, all ozone-induced particle destabilization phenomena observed here were primarily the result of pH decreases, pH increases, or precipitation of CaCO3. Because these changes are produced more effectively and at lower cost with chemical addition, aeration, or both, they must be considered in properly gauging the true benefits of ozonation with respect to particle removal. Includes 23 references, tables, figures.