Abstract. Multiarm star polymers, with a hyperbranched poly(ethyleneimine) (PEI) core and poly("-caprolactone) (PCL) arms end-capped with acetyl groups were synthesized by ring-opening polymerization of "-caprolactone from PEI cores of different molecular weight. These star polymers were used as toughening agents for epoxy/anhydride thermosets. The curing process was studied by calorimetry, thermomechanical analysis and infrared spectroscopy. The final properties of the resulting materials were determined by thermal and mechanical tests. The addition of the star polymers led to an improvement up to 130% on impact strength and a reduction in the thermal stresses up to 55%. The structure and molecular weight of the modifier used affected the morphology of the resulting materials. Electron microscopy showed phase-separated morphologies with nano-sized fine particles well adhered to the epoxy/anhydride matrix when the higher molecular weight modifier was used.