The effect of reaction time on the trihalomethane and nonpurgeable total organic-halide formation potentials was determined by chlorinating water samples from the Mississippi, Missouri, and Ohio Rivers. Samples were collected for three seasons at 12 locations on the Mississippi from Minneapolis, Minnesota, to New Orleans, Louisiana, and on the Missouri and Ohio 1.6 kilometers above their confluences with the Mississippi. Both types of compounds formed rapidly during the initial stages of the reaction-time period, with formation rates decreasing with time. The ratio of the nonpurgeable total organic-halide and trihalomethane concentrations decreased with time, with the nonpurgeable total organic-halide compounds forming faster during the first stages of the time period and the trihalomethane compounds forming faster during the latter stages of the time period. Variation with distance along the Mississippi River of the formation rates approximately paralleled the variation of the dissolved organic carbon concentration, indicating that the rates of formation, as well as the concentrations of the compounds formed, depended on the dissolved organic carbon concentration.