Because of their nanoporous structure and large surface area, carbon aerogels have high potential for improving the material properties of polymer-based composites. In the present study, the mechanical properties and toughness of epoxy polymers modified with an aerogel content of 0.0-0.5 wt% were considered. Experimental results showed that the stiffness and strength of the carbon aerogel toughened polymers steadily increased with the carbon aerogel content. The glass transition temperature of the unmodified epoxy polymer was 147 A degrees C, and it was not appreciably affected by the addition of carbon aerogels. Blending the carbon aerogels with the epoxy polymer led to an appreciably improvement in the fracture performance of the resulting composites. For example, the fracture energy of the unmodified polymer was 125 J m(-2), whereas that of an epoxy polymer reinforced with a carbon aerogel content of 0.3 wt% was 255 J m(-2). The mechanisms responsible for the toughness enhancement were identified by studying the fracture surfaces using field emission gun scanning electron microscopy. Crack pinning, crack deflection, interfacial debonding, and plastic void growth were the main toughening mechanisms in the carbon aerogel toughened epoxy polymers.