Inactivation kinetics with continuously applied ozone were approximately exponential with respect to time and concentration. The most resistant of the tested viruses was human rotavirus and the most sensitive f2 coliphage. The disinfectant was most reactive at low pH and viral sensitivity was increased ten-fold in the presence of 0.5M sodium bicarbonate. Temperature, the presence of added peptone nor suspended solids greatly influenced inactivation, but ultrasonic treatment did, whether applied before or during ozonation. The chemistry of ozone and its mechanism of action is discussed.