Within this study the influence of adding 5 vol % of silica nanoparticles, obtained via a sol-gel process, on an Araldite-F epoxy was investigated. To characterize toughening effects, compact tension specimens were used to obtain K-IC and G(IC). Additionally, tensile strength and E-Modulus were Measured as well as differential scanning calorimetry and dynamic mechanical thermal analysis were carried out to evaluate the influence on the thermal properties of the epoxy because of addition of the particles. Electronic microscopy was used to check dispersion quality and fracture surfaces, in transmission mode and scanning mode, respectively. The addition of 5 vol %. silica-nanoparticles could improve the stiffness and the toughness of an epoxy resin at the same time. (c) 2006 Wiley Periodicals, Inc.