Abstract: A biodegradable and biocompatible polyester comprising (C.sub.3-10) alkylene carbonic acid ester units, each alkylene group being a C.sub.3 -alkylene group having 1 oxy substituent or a (C.sub.4-10) alkylene group having 2-8 oxy substituents, each of the oxy substituents occurring individually as a hydroxyl group or as a derivatized hydroxyl group comprising an ester or an ortho ester or an acetal residue.The polyesters may be used as matrices for the sustained release of pharmacologically active compounds, e.g. peptides or proteins, in the form of microparticles or implants.