The photoinitiated oxidative reactions of 2,4,6 trichlorophenol (2,4,6 TCP) and 2,4,5 trichlorophenol (2,4,5 TCP) are studied on the titania photocatalyst Degussa P25. On this catalyst 2,4,6 TCP is used to confirm two distinct oxidative mechanisms that are triggered at different light-excitation wavelengths. A charge-transfer mechanism occurs at sub-bandgap energies of the photocatalyst leading to a phenoxyl radical product and an oxidative mechanism occurs at the bandgap leading to a semiquinone radical product. The wavelength dependence of these two mechanisms is discussed.