A bisphenol-A epoxy resin cured with triethylenetetramine was modified with the addition of a polyamide oligomer and a small amount of montmorillonite. Compositions with different amounts of modifiers were obtained and tested for their impact strength, flexural strength, and resistance to crack propagation. The latter was assessed by measuring the critical stress intensity factor in a three-point bending mode. Scanning electron microscopy was used to examine the sample fracture surfaces. It was found that the addition of 2% montmorillonite or 5% polyamide resulted in the best improvement of the impact strength and the critical stress intensity factor relative to the unmodified epoxy resin. However, the flexural strength and toughness measured under three-point bending mode was found to increase to a lesser extent. Hybrid compositions containing specific combinations of both modifier and nanofiller not only exhibited a higher impact strength and resistance to crack propagation but also displayed a synergistic effect in relation to the fracture energy. The results indicate that the improvement in mechanical properties of the epoxy resin was due to the formation of a heterogeneous morphology resulting from phase separation of the polymeric modifier. From the scanning electron microscopy and thermal analysis, it appears that the toughening may arise from chemical reactions that have taken place between the epoxy resin and the polymeric modifier, which was partially solubilized in the resin matrix. (C) 2008 Wiley Periodicals, Inc.