Inactivation studies with ozone under different environmental conditions were conducted to determine the effect on process efficiency in various continuous flow systems. The effects of pH, mixing, density or organisms and organic matter were investigated in terms of microorganism inactivation. Experiments were also performed to demonstrate the effect of ozone bubbles on disinfection. Results indicated that ozone residual is the controlling factor in the inactivation of microorganisms. Also, based on the conceptual model proposed, the effectiveness of ozone disinfection appears to be limited by the overall rate of ozone transfer into the bulk liquid rather than by direct contact with ozone gas bubbles.