Graphene oxide (GO) can be prepared economically from graphite. The epoxide and hydroxyl groups on provide many opportunities for attaching functional groups to the surface of GO. These can aid its dispersion in polymers and grafting to reactive systems. We show that very low levels of functionalized GO can increase the fracture toughness of brittle thermosets like epoxies and unsaturated polyesters. A microcrack mechanism is proposed based on microscopy of the fracture surfaces. Due to the stress concentration by graphene nanosheets, microcracks may be formed to absorb the fracture energy. However, above a certain graphene concentration, the coalescence of microcracks appears to facilitate crack propagation, lowering the fracture toughness. H. Kim, A.A. Abdala, C.W. Macosko, Graphene/polymer nanocomposites, Macromolecules 43 (2010) 6515-6530. Y. Park, Y. Qian, C. Chan, T. Suh, M.G. Nejhad, C.W. A. Stein, Adv. Funct. Mater. 2014, 25, 575–585,