The Metropolitan Water District of Southern California recently changed its primary disinfectant from free chlorine to chloramines in order to help its member agencies control trihalomethanes (THMs). The effects of blending chloraminated and chlorinated waters were studied by developing blend‐residual curves to simulate the systems involved. Since these curves were found to compare well with the printouts of a computer model designed to estimate desirable blends, it was concluded that the computer model could be used to predict acceptable blends and to anticipate bacteriological, THM, and taste‐and‐odor problems, thus eliminating the need to determine each case experimentally.