Block copolyestercarbonates may be prepared by first conducting a reaction between at least one of resorcinol or an alkyl- or haloresorcinol and at least one aromatic dicarboxylic acid dichloride, preferably isophthaloyl dichloride, terephthaloyl dichloride or a mixture thereof, to produce a hydroxy-terminated polyester intermediate, and then conducting a reaction of the intermediate with a carbonate precursor, preferably in the presence of a dihydroxy compound such as bisphenol A. The products have excellent physical properties, including a high degree of weatherability. They may be blended with other polymers such as polycarbonates, poly(alkylene carboxylates), polyarylates, polyetherimides, and addition polymers to improve the weatherability thereof.