Irradiation of methyl vinyl ketone (MVK)/acrylonitrile(AN) mixtures gives facile copolymerization accompanied by small molecule formation. A free radical polymerization mechanism is supported by copolymer formation, absence of solvent polarity effect, and inhibition by TEMPO. Small molecules, presumably cyclobutanes, are favored in dilute solutions and form competitively with copolymer. The excited triplet MVK biradical is identified as the initiating species.