||Monochloramination has been commonly applied as an alternative to chlorination for the purpose of reducing the formation of disinfection by-products while sufficient microbial inactivation could still be achieved. This process is practical since it can be accomplished by simple modifications of the existing chlorine feeding system and its relatively low operational cost compared to other alternatives. However, inactivation kinetics of monochloramination is not well understood. Vast discrepancy between laboratory studies and field practices has been revealed (Wolf and Olson, 1984). Besides, studies have demonstrated that the formation of haloacetic acids (HAAs) in monochloramination process is also of concern (Cowman and Singer, 1996). This study investigated factors that possibly affect the inactivation performance and the HAA formation in monochloramination, including monochloramination applying techniques, chlorine to ammonia nitrogen mass ratios, and water quality. To achieve this goal, both synthetic solutions and real wastewater effluents were monochloraminated in well-mixed reactors under different experimental conditions. Samples were periodically withdrawn and subjected to analyses for evaluating the time-dependent inactivation performance and/or HAA formation. Results of this study showed that the “preammoniation” approach and/or a higher chlorine to ammonia nitrogen ratio could achieve better inactivation, but might also lead to higher yields of HAAs. In the case of preammoniation, the short existence of free chlorine significantly affected the overall inactivation by introducing the initial kill (or synergism) and the HAA formation by influencing the speciation as well as the total concentration of HAAs. High bromide content did not show any significant effect on the inactivation performance but could increase the total HAA concentration and shift HAAs towards the bromine-containing species. In addition, the differences in the dose-response behavior between laboratory studies and field practices were examined. A reliable and convenient approach of conducting inactivation tests was proposed. The mechanisms and the chlorine chemistry contributing to HAA formation in complex wastewater environment were discussed.