The complete photocatalytic oxidation of C2H4 With O-2 into CO2 and H2O has been achieved on ultrafine powdered TiO2 photocatalysts and the addition of H2O was found to enhance the reaction. The details of the photocatalytic reaction have been studied by IR, ESR, and analysis of the reaction products. UV irradiation of the photocatalysts at 275 K led to the photocatalytic oxidation of C2H4 with O-2 into CO2, CO, and H2O. The large surface area of the photocatalyst is one of the most important factors in achieving a high efficiency in the photocatalytic oxidation of C2H4. The photo-formed OH species as well as O-2(-) and O-3(-) anion radicals play a significant role as a key active species in the complete photocatalytic oxidation of C2H4 with O-2 into CO2 and H2O.