The spontaneous photochemical copolymerization of 2,3-dimethyl-1,3-butadiene (DMB) and acrylonitrile was investigated at room temperature. The accompanying cycloaddition was also studied. The free radical copolymerization proceeded slowly at several monomer concentrations under UV irradiation. Higher yields of copolymer were observed in the presence of benzophenone as sensitizer, but the cycloadduct yield was almost unaffected. Maximum copolymer yield was obtained at 1:1 feed ratio. The cycloadducts consisted mostly of cyclobutane adducts as well as the Diels-Alder adduct, and the maximum yield of cycloadduct was obtained at approximately 75 mol % DMB in the feed. A mechanism involving the singlet and triplet excited states of DMB is proposed, in which the initiating species is proposed to be the 6-cyano-2-hexene-1,6-diradical, formed by reaction of the triplet excited s-trans DMB with acrylonitrile.