Abstract We herein report the visible-light-induced photocatalytic degradation of ethylene by a C-doped TiO2 catalyst synthesized by the sol–gel method. The effects of key parameters such as visible light intensity, temperature, and feed composition (ethylene, oxygen, and water vapor) on the photocatalytic reaction rate were evaluated. Our observations, along with experimental results, indicated that the oxidation rate of ethylene improved significantly with an increase in visible light intensity, temperature, and oxygen and ethylene concentrations. Under all experimental conditions, approximately 96–753ppmv of ethylene, which was adsorbed onto the photocatalyst surfaces (C-doped TiO2) were stoichiometrically oxidized to CO2. High temperature was found to improve the oxidation rate, which could be attributed to an increase in the reactivity of the heterogeneous catalyst and also a hostile adsorption of water on the catalyst. We found that the oxidation rate of ethylene was suppressed considerably with an increase in the water vapor concentration from 547 to 15,000ppmv at two specific temperatures (303 and 318K). This observation can be explained by the law of adsorption, since polar water molecules have higher adsorption affinity on a polar catalyst surface than do nonpolar ethylene molecules. We used a Langmuir–Hinshelwood (L–H) model with explicit temperature dependence for simulating the entire set of experimental rate data. The rate law was expressed as follows to account for the obtained results: r=Iαk′exp-EaRTKe′exp-ΔHe/RTTCe1+Ke′exp-ΔHe/RTTCe+Kw′exp(-ΔHw/RTT)CwKO2′exp-ΔHO2/RTTCO21+KO2′exp-ΔHO2/RTTCO2 Based on the L–H model, we determined the adsorption enthalpies of ethylene, water vapor, and oxygen on the C-doped TiO2 catalyst.