Epoxy formulations containing mixtures of two different size distributions of silica-based particles (micron-size and nanometer-size) were explored for possible synergistic toughening effects. The influence of bimodal particle size distribution and silica particle content on the glass transition temperature (T-g), coefficient of thermal expansion (CTE), Young's modulus (E), tensile yield stress (sigma(y)), and fracture toughness were investigated. Interestingly, fracture toughness improved by approximately 30% when mixtures of microparticles and nanoparticles were used. The origins for these improvements in toughness were explored using scanning electron microscopy (SEM) and transmission optical microscopy (TOM). These techniques revealed that the toughness improvements were due to the debonding of the microparticles and subsequent plastic void growth of the matrix, as well as more void growth due to a high fraction of debonded nanoparticles. The improvements in toughness were higher when the volume fraction of microparticles was less than the volume fraction of nanoparticles (overall filler content was fixed at 10 vol% in this study). The increased toughness in epoxies with mixtures of particles can be explained by summing the contribution of microparticle-induced matrix void growth, nanoparticle-induced matrix void growth and nanoparticle-induced matrix shear banding. (c) 2013 Elsevier Ltd. All rights reserved.