In the present study, the decomposition rates of carbon tetrachloride (CCl ) and 2,4-dichlorophenol (2,4-DCP) in water by the ultraviolet (UV) light irradiation alone and H O /UV were experimentally investigated. The detailed experimental studies have been conducted for examining treatment capacities of the two different ultraviolet light sources (low and medium pressure Hg arc) in H O /UV processes. The low or medium UV lamp alone resulted in a 60%–90% decomposition of 2,4-DCP while a slight addition of H O resulted in a drastic enhancement of the 2,4-DCP decomposition rate. The decomposition rate of 2,4-DCP with the medium pressure UV lamp alone was about 3–6 times greater than the low pressure UV lamp alone. In the direct photolysis of aqueous CCl , the medium pressure UV lamp had advantage over the low pressure UV lamp because the molar extinction coefficient of CCl at shorter wavelength (210–220 nm) is about 20 to 50 times higher than that at 254 nm. However, adding H O to the medium pressure UV lamp system rendered a negative oxidation rate because H O acted as a UV absorber being competitive with CCl due to negligible reaction between CCl and OH radicals. The results from the present study indicated significant influence of the photochemical properties of the target contaminants on the photochemical treatment characteristics for designing cost-effective UV-based degradation of toxic contaminants.