Epoxy-silica nanocomposites were produced by dispersing silica-organosol particles in TGDDM/DDS resin mixtures, The resulting materials were investigated in terms of chemical interactions, curing behavior and mechanical and fracture properties. A reaction between epoxy groups and silanol groups present on the surface of the silica phase was detected, leading to an increased interfacial adhesion. The curing behavior of the epoxy matrix was not adversely affected by the inorganic phase. A conspicuous increase of modulus and yield strength was found by increasing the silica content. The reinforcement factors, i.e. the normalized modulus and yield strength, were found to be constant over a wide temperature range (25-180 degrees C). The activation volume and activation energy for yielding were found to decrease with silica content in the opposite manner, indicating an increased constraint of the structural segments involved in the yielding process caused by interfacial interactions between the silica particles and the epoxy matrix. Fracture mechanics tests showed that the addition of silica nanoparticles up to 10 wt% brings about a considerable enhancement in fracture toughness and an increase in the critical crack length for the onset of crack propagation. This enhancement in toughness is larger than that achieved until now with micro-sized particles. (c) 2005 Elsevier Ltd. All rights reserved.