Alternating copolymers of beta -methylstyrene and maleic anhydride were prepared by free-radical-initiated polymerization in bulk and in toluene as a solvent. The reactivity ratios k//1//c/k//1//2 and k//2//c/k//2//1 were calculated from the change of copolymerization rate with a monomer feed at a constant total monomer concentration according to the generalized model of Y. Shirota and coworkers. Termination occurs predominantly by homotermination of beta -methylstyrene macro free radicals, k//t//2//2, but the cross termination k//t//2//1 is also operative.