Trihalomethane (THM) formation was determined by gas chromatography in water disinfected with chlorine dioxide (ClO2) only or ClO2 combined with chlorine (Cl2), to lay a basis for water disinfected with ClO2. Results showed no THM was produced in water disinfected with ClO2 only if without existence of bromine (Br) ion in it; certain amount of bromoform could be formed with existence of bromine ion, and THM formation could be inhibited by ClO2 and Cl2, with a ratio of greater than 1 between them. Light irradiation could decrease the amount of THM produced whether in disinfection with ClO2 or combination with ClO2 and Cl2. No significant effect of pH value of water on THM formation was observed, but pH should be controlled under neutral or slightly acidic to ensure the concentration of ClO2 in water.