Copolymers are prepared by the interaction of a melt mix comprising a high molecular weight poly(alkylene arylate) resin with a high molecular weight polycarbonate or polyester carbonate and a catalyst, such as diphenyl phosphate. The copolymers are tough and are useful as molding compounds and are capable of maintaining dimensional stability at temperatures approaching their singale glass transition temperature. The copolymers are characterized by having substantially no content of polyester homopolymer and a ratio of arylate linkages to dihydric phenol arylate linkages of not substantially less than 20:1.