An investigation has been made of the cadmium photosensitized reactions of ethylene and of ethylene-hydrogen mixtures. With ethylene alone the quantum yield of the reaction is very low, viz., about 0.01. With ethylene-hydrogen mixtures the quantum yield is about 0.5 to 0.7 depending upon the conditions. The products of the reaction are methane, ethane, propane, butane, propylene, and higher hydrocarbons. The mechanism of the reaction is discussed and compared with that of the mercury photosensitized reaction. It is concluded that the main steps of the reaction in the presence of hydrogen are[Formula: see text]together with radical recombination reactions, and reactions of the type[Formula: see text]