To clarify the mechanism of the spontaneous copolymerization of styrene with acrylonitrile, the corresponding reaction of its analog, indene, was investigated. The spontaneous free radical copolymerization of indene with acrylonitrile was promoted by Lewis acids. Spontaneous copolymerization rates comparable to those of styrene with acrylonitrile were found. For indene, the Mayo [pi(4) + pi(2)] initiation mechanism, initially postulated for the spontaneous polymerization of styrene, cannot occur. Therefore the Flory mechanism, postulating a tetramethylene biradical initiating species, is proposed.