The author of this letter discusses the use of ozone as a primary disinfectant with postdisinfection achieved through the use of preformed monochloramine. The preformation of a strong stock solution of monochloramine under optimum conditions gives a chemically well defined solution of pure monochloramine and no detectable amounts of free chlorine, dichloramine, or trichloramine, which are responsible for tastes and odors. Since the introduction of the use of preformed monochloramine there has been an improvement in taste and odor and no trihalomethanes have been found in the water.