This study describes the miscibility phase behavior in two series of biodegradable triblock copolymers, poly(L-lactide)-block-poly(ethylene glycol)-block-poly(L-lactide) (PLLA-PEG-PLLA), prepared from two di-hydroxy-terminated PEG prepolymers (M-n = 4000 or 600 g mol(-1)) with different lengths of poly(L-lactide) segments (polymerization degree, DP = 1.2-145.6). The prepared block copolymers presented wide range of molecular weights (800-25,000 g mol(-1)) and compositions (16-80wt.% of PEG). The copolymer multiphases coexistance and interaction were evaluated by DSC and TGA. The copolymers presented a dual stage thermal degradation and decreased thermal stability compared to PEG homopolymers. In addition, DSC analyses allowed the observation of multiphase separation; the melting temperature, T-m, of PLLA and PEG phases depended on the relative segment lengths and the only observed glass transition temperature (T-g) in copolymers indicated miscibility in the amorphous phase. (c) 2006 Elsevier B.V. All rights reserved.