High molecular weight poly(ester carbonate)s have been prepared by a two-step chain-extension reaction performed on oligomeric alpha,omega-dihydroxy-terminated poly(1,3-propylene succinate) in turn obtained by thermal polycondensation of excess 1,3-propanediol with succinic acid. The new polymers have a biodegradable backbone and derive from renewable sources. Therefore, they have a potential as environment-friendly materials.