We report on a synthetic method where polyanhydride is used as starting material and the ester monomers are inserted through complete esterification, leading to ail alternating ester-anhydride copolymer. The molar ratio of ricinoleic add.(RA) and sebacic acid (SA) was optimized until- polysebacic acid is completely converted to carboxylic acid-terminated RA-SA and RA-SA-RA ester-dicarboxylic acids. These, dimers and trimers were activated with acetic anhydride, polymerized. under heat and vacuum to yield alternating RA-SA- copolymer. The fesulting alternating poly(ester-anhydride) have the RA at regular intervals. The regular occurrences Of RA side chains prevent anhydride interchange, enhancing hydrolytic stability, which allows storage of the polymer at room temperature.