Epoxy microspheres, with average diameters in the order of 4-5 mu m, were first produced as insoluble gel resulting from the reactions of a bisphenolic epoxy resin with an aromatic hardener in solution. They were then used as reactive flow modifiers and toughening agents for bisphenolic epoxy resins with same chemical composition. Thermal analysis, rheological measurements, and impact tests were performed to assess their ability to modify the viscosity of the uncured resin and to determine the effects on the cured resins with respect to the glass transition temperature and impact strength. Two similar epoxy systems modified with the addition of different amounts of microspheres preserve the glass transition temperature of the cured unmodified epoxy. The addition of microspheres is responsible for secondary transition. The addition of a small fraction of microspheres does not affect the reactivity of the epoxy resin, while the inclusion of a higher content leads to a noticeable reduction in the gel time of the epoxy systems, indicating that microspheres are not inert fillers. Finally, a limited enhancement of the toughness of the epoxy resin was observed, as a consequence of strong interactions between the matrix and the microspheres. (c) 2006 Wiley Periodicals, Inc.