The crosslinking of internally plasticized polyvinyl chloride–stearate copolymer by irradiation with high-energy electrons was studied. Based on gel yields and swelling as a measure of relative crosslinking, the effect of added tetra-functional monomers, and of carbon black and silica fillers are discussed. Crosslinking by dicumyl peroxide a t 170°C. even in the presence of a stabilizer, causes concurrent decomposition (dehydrochlorination) which is avoided in the radiation-crosslinking process. The heat stability of the copolymer is not impaired by radiation crosslinking under the proper conditions. Results of tensile measurements a t 25 and 150°C. are given for the radiation-crosslinked copolymer with and without filler reinforcement.