Copolymerization of cyclic phosphonites (1a-c) with methacrylic and acrylic anhydrides (2a and 2b, respectively) has been investigated. The results show that a 1:1 mixture of monomers produces an alternating copolymer with one part of the repeating unit coming from the ring opening of monomer 1 and the other part from the ring closing of 2 (ring-opening-closing alternating copolymerization (ROCAC)). In the copolymerization of 1 with 2a under some conditions, the quantitative cyclization of 2a took place to form an alternating copolymer of 3a, whereas a small amount of pendent unsaturation was observed in the copolymerization of 1 with 2b