The stabilities in water of several new N-halamine disinfectants have been measured. Variables in the study included pH, temperature, and water quality (demand-free and synthetic-demand). An inverse relationship exists between the stabilities of the compounds and their efficacies against Staphylococcus aureus. The most stable compounds of those studied, 1,3-dichloro-4,4,5,5-tetramethyl-2-imidazolidinone ( A) and 3-chloro-4,4-dimethyl-2-oxazolidinone ( I), are possible candidates as water disinfectants for high-temperature applications such as hot tubs and cooling towers. While the N-bromamines studied were generally less stable but more biocidal than their chloramine analogs, the mixed bromochloro compound 1-bromo-3-chloro-4,4,5,5-tetramethyl-2-imidazolidinone ( ABC) provided rapid initial disinfection due to the Br moiety and slower long-term disinfection due to the Cl moiety. The disinfectant of choice is dependent upon the envisioned application.