A miscibility and phase behavior study was conducted on poly(ethylene glycol) (PEG)/poly(l-lactide-epsilon-caprolactone) (PLA-co-CL) blends. A single glass transition evolution was determined by differential scanning calorimetry initially suggesting a miscible system; however, the unusual T-g bias and subsequent morphological study conducted by polarized light optical microscopy (PLOM) and atomic force microscopy (AFM) evidenced a phase separated system for the whole range of blend compositions. PEG spherulites were found in all blends except for the PEG/PLA-co-CL 20/80 composition, with no interference of the comonomer in the melting point of PEG (T-m=64 degrees C) and only a small one in crystallinity fraction (X-c=80% vs. 70%). However, a clear continuous decrease in PEG spherulites growth rate (G) with increasing PLA-co-CL content was determined in the blends isothermally crystallized at 37 degrees C, G being 37 mu m/min for the neat PEG and 12 mu m/min for the 20 wt % PLA-co-CL blend. The kinetics interference in crystal growth rate of PEG suggests a diluting effect of the PLA-co-CL in the blends; further, PLOM and AFM provided unequivocal evidence of the interfering effect of PLA-co-CL on PEG crystal morphology, demonstrating imperfect crystallization in blends with interfibrillar location of the diluting amorphous component. Significantly, AFM images provided also evidence of amorphous phase separation between PEG and PLA-co-CL. A true T-g vs. composition diagram is proposed on the basis of the AFM analysis for phase separated PEG/PLA-co-CL blends revealing the existence of a second PLA-co-CL rich phase. According to the partial miscibility established by AFM analysis, PEG and PLA-co-CL rich phases, depending on blend composition, contain respectively an amount of the minority component leading to a system presenting, for every composition, two T-g's that are different of those of pure components. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 111-121