Trihalomethanes (THM's) are formed during the chlorination of waters as a result of the reaction between chlorine and naturally occurring aquatic organic compounds and humic substances. The effect of bromide ion at various concentrations of chlorine, pH, and reaction time were studied. All of these interact and influence the formation and distribution of individual trihalomethanes (THM's). Addition of bromide, longer reaction time, higher pH, and higher chlorine concentration were all found to increase the yield of trihalomethanes. The chlorine concentration corresponding to the break point resulted in THM yield less than that observed for the chlorine concentrations on either side of the break point. The phenomenon is as yet unexplained.