Dendritic hyperbranched polymers (HBPs) show outstanding performance as tougheners in epoxy resins. The critical strain energy release rate, G(Ic), of a DGEBF resin is increased by a factor of 6 from 120 J/m(2) to 720 J/m(2) using only a 5% modifier weight content. This does not lead to a decrease in resin stiffness or glass transition temperature and the favourable processability of epoxy resins is fully maintained without requiring the use of solvents. Toughness properties are induced by a finely-dispersed particulate structure which is obtained by a controlled phase separation process during resin curing. A strong mechanical phase interaction is ensured by chemical bonding through reactive epoxy groups grafted onto the HBP. By tailoring the HBP＇s epoxy functionality and polarity to control reactivity, phase separation, final morphology and phase interactions, this toughening system is potentially applicable to a variety of thermoset resins. With this in view, the influence of the chemical architecture of the HBP modifiers on phase separation, processability, properties and toughening mechanisms is investigated.