New hyperstar polymers (HSP) based on a hyperbranched polyester core (HBP-OH) and PMMA or P(MMA-b-HEMA) block copolymer arms have been prepared for use as nanoscale soft particle-like additives in an epoxy matrix cured cationically either by UV light or thermally. The effects of the additive as well as the curing process on the epoxy group conversion, gel content, morphology and the thermal and viscoelastic properties of the cured materials were evaluated. It was found that in both curing processes the non-reactive hyperstar HBP-PMMA resulted in phase-separated materials with low effect on flexibilization whereas by use of HBP-P(MMA-b-HEMA) the OH groups in the HEMA units allowed chemical bonding of the HSP to the epoxy network and hindered phase separation. The epoxy group conversion was enhanced in thermally cured samples but significantly decreased in photocured ones showing the most effective flexibilization demonstrated as reduction in T(g), whereas in both cases, high gel contents have been achieved using the reactive HSPs. Thus, the increased temperature in the thermal curing process enhances the effectiveness of the HSPs as multifunctional crosslinkers leading to higher curing rates and crosslinking density, whereas in photocuring the flexibilization effect can be enhanced by having high gel content at lower epoxy group conversion. (C) 2011 Elsevier Ltd. All rights reserved.