Copolymers of (R)-3-hydroxybutyric acid (HB) and epsilon-caprolactone (CL) with a composition ranging from 28 to 81 mol % of HB were synthesized by transesterification of the corresponding homopolymers in solution in the presence of 4-toluenesulfonic acid. The copolyesters were characterized with regard to their molecular weights, thermal properties, molar compositions, and average block length of repeating units by gel permeation chromatography (GPC), differential scanning calorimetry, H-1 NMR, and C-13 NMR, respectively. Random and microblock copolymers could be obtained depending on experimental conditions, with weight-average molecular weights of up to 20 000. The glass transition temperature decreased from 2 to -42 degreesC as the CL content was increased from 0 to 72 mol %. The melting temperature (T-m) of the PCL phase decreased from 70 to 46 degreesC as the HB content changed from 0 to 47 mol %, while the T-m of the PHB phase decreased from 177 degreesC to 163 degreesC as the CL content changed from 0 to 72 mol %. Matrix-assisted laser desorption ionization time-of-flight mass spectra of GPC fractionated samples allowed us to ascertain that copolymers rich in HB units have mostly hydroxyl and carboxyl end groups, while copolymers rich in CL units have mostly tosyl and carboxyl end groups.