The yields of gases evolved from three types of polyethylene and ethylene-propylene copolymer during radiation were precisely measured after gamma ray irradiation under vacuum over a wide range of temperatures (−196 to 200°C). For all polymers the major gas evolved was H 2 and the minor products were C 1 , C 2 , C 3 hydrocarbons and the oxidation compounds CO 2 and CO. The total gas yield increased with an increase in the irradiation temperature, but the ratio of the yields among the gas components was not greatly changed. The H 2 would originate from H-bond scission and the concomitant formation of crosslinks and double bonds in the polymer chains. The minor products of C 1 , C 2 , C 3 hydrocarbons were products of chain scission at the chain ends, including branched chains, and the oxidation compounds of CO and CO 2 were the products formed by reactions of oxygen remaining trapped in the polymer matrix. The yield of H 2 increased with increasing irradiation temperature, which is closely related to the molecular motions of the polymer chains during irradiation.