Cyanuric acid, used commonly in swimming pools to stabilize chlorine, was investigated in an attempt to control trihalomethane (THM) formation in filtered river water. At 7.5:1 molar ratio, applied cyanuric acid to applied chlorine, THM formation was reduced by 29 per cent. The reduction diminished with decreasing molar ratio and increasing pH, both of which promoted higher free chlorine residuals. A model was employed to differentiate between free chlorine and chlorinated isocyanurates. At 7.5:1 in bromide-spiked water, THM formation was reduced only 9 per cent and formation of brominated THM species was favored; increasing molar ratio favored increasing bromoform formation. The data could be explained, in part, by the classical haloform reaction. Cyanuric acid probably holds little promise for THM control in drinking water treatment.