Polyesterpolycarbonates of formula (I) ##STR1## wherein a is an integer from 2 to 300; R.sub.1 and R.sub.2, which can be the same or different, are each a polyester residue of formula (II) ##STR2## with x and y being integers from 0 to 100, and the ratio of (x/x+y)*100 being between 0 and 100, with the proviso that x and y are not 0 at the same time; R.sub.3 and R.sub.4 being aliphatic hydrocarbon residues of 1 to 4 carbon atoms; R.sub.5 being an aliphatic hydrocarbon residue having from 2 to 18 carbon atoms or a cycloaliphatic hydrocarbon residue having from 3 to 8 carbon atoms; or a polyoxyalkylene residue of formula (III): ##STR3## wherein R.sub.6 is hydrogen or methyl, n is an integer from 1 to 3 and m is an integer from 1 to 200; and the two --R.sub.3 --COO and --R.sub.4 --COO groups are randomly distributed in the polyester residue, with the polyesterpolycarbonates of formula (I) having an intrinsic viscosity of greater than 0.45 dl/g when measured in chloroform at 32.degree. C. and being useful as bioerosible matrices for the controlled release of drugs.