Current regulations on trihalomethane (THM) concentrations have led many utilities to explore the use of other oxidants and disinfectants to partially or completely replace chlorine in their treatment schemes. A mixed oxidant solution, generated electrolytically from brine, by the "MIOX" system, appears to be an attractive alternative to chlorine with respect to disinfection and THM formation. However, little is known about the chemical composition of this mixed oxidant solution except that which is based on electrochemical theory. The primary objective of this investigation was to evaluate the formation of THMs by MIOX and to compare the results with those obtained in control experiments using equivalent amounts of sodium hypochlorite (NaOCl). The secondary objective of this investigation was to characterize the mixed oxidant solution as to its chemical composition. Model waters of well-known chemical composition as well as several real waters of varying quality were treated with MIOX and with NaOCl under identical experimental conditions, and the extent and rate of THM formation was examined. The results showed that THM formation in MIOX-treated waters was essentially the same as that in NaOCl-treated waters. Little insight as to the chemical composition of the mixed oxidant solution was gained in this investigation.