Boltorn H30 hyperbranched polyester has been modified by acylation with 10-undecenoyl chloride to obtain HBPs with different degree of modification. These HBPs have been used as reactive modifiers, in a proportion of 5 and 10%, of DGEBA/MHHPA/BDMA formulations. The materials obtained have been characterized and their mechanical properties evaluated. Depending on the degree of modification of the HBP, homogeneous or phase separated materials were obtained, which influence the properties. When the degree of modification of the HBP was a 76%, a 4-fold increase in impact resistance was achieved without sacrificing thermomechanical characteristics, thermal stability or processability. This has been attributed to the regular microphase separation achieved and to the good interfacial interaction of the microparticles with the epoxy matrix by formation of covalent bonds between the remaining hydroxyl groups at the shell of the HBP and the anhydride as the curing agent. (c) 2012 Elsevier Ltd. All rights reserved.