The phase diagrams for blends of semicrystalline poly(epsilon-caprolactone) (PCL) with amorphous polystyrene (PS) were determined and it was noticed that when molecular weight increases, the critical composition diminishes while its temperature increases. That is the same effect that produces the increase in the polydispersity of PS. From the fitting of phase diagrams of blends of PCL with oligomers and homopolymers of PS, taking into account the polydispersity, it was possible to conclude that the miscibility in these systems is due only by entropic effects. The crystalline fraction as well as the interlamellar spacing increased with crystallization temperature while the volume fraction of the amorphous PCL in the interfibrillar regions increases for the pure PCL and in its blends with low concentration of PS. Increasing the mass fraction of PS increases the average long spacing (L) till a weight fraction of 60%, for higher mass fractions L is almost constant indicating the interfibrillar segregation of PS. The PCL lamellar morphology is not affected when the molecular weight of PS employed is similar or higher than the molecular weight of PCL. If the PS has a higher molecular weight its penetration into the structure is inhibited. (C) 2016 Society of Plastics Engineers