Three different types of polyester-polyether copolymers were synthesized by PEG initiated ring-opening polymerization of epsilon-caprolactone : an A-B and an A-B-A block copolymer and an (A)(2)-B star copolymer, where A is a poly(epsilon-caprolactone) (PCL) and B is a poly(ethylene glycol) (PEG) block. The monomer to initiator ratio was varied to obtain copolymers with different PCL block length and weight fraction ranging from 68-85 wt%. These PCL-PEG copolymers were characterized by means of g.p.c., n.m.r., d.s.c. and WAXD.In all types of copolymer the PCL constituent crystallizes first when cooling from the molten state. This leads to significant undercooling and imperfect crystallization of the PEG block. The mutual influence between PCL and PEG constituents is significantly stronger in the A-B-A block copolymers, having a central PEG block, and in the (A)(2)-B star copolymers.The effect of the PCL/PEG ratio on the melting and crystallization behaviour of the copolymers was investigated. The results obtained demonstrate that the thermal properties and morphology are affected significantly by the chain length of the PCL and PEG constituents in the copolymers and by the type of copolymers (sequence of the different blocks).