Trihalomethane (THM) in drinking water is formed by chlorination of humic substances. In this study, the rates of THM formation in aqueous solution of humic acid were examined under various conditions. The following rate equation was obtained empirically. [THM] = k (pH − a)[TOC][Cl 2] 0 m t n . Here, [THM] is the concentration of total THM after t h, [TOC] and [Cl 2] 0 are the concentrations of total organic carbon and chlorine dose, k is the rate constant and a, m and n are parameters. The values of k, a, m and n for humic acid as reagent were obtained as 8.2 × 10 −4 ( l m mg − m h − n ), 2.8, 0.25 and 0.36, respectively. The activation energy was obtained as 37 kJ mol −1. Further, it was proved that the above equation could be applied to the rates of THM formation from precursors in actual river and lake waters.