AbstractPolystyrene nanofibres reinforced with cellulose nanocrystals have been produced by electrospinning. Two different orientations of the nanofibres were produced, namely aligned and random, and these nanofibrous interleaves were investigated both as-spun and after thermal treatment. Aligned nanofibres exhibited a 2500% increase in their Young’s modulus compared to the randomly orientated materials, while the ultimate tensile strength increased by up to 300%. It is also demonstrated that crazing occurs in the nanofibres, which is thought to enhance the fracture properties of the materials. Interleaves of both orientations were then included in a carbon fibre-reinforced polymer laminate. The fracture toughness of the laminate was determined under both mode I and mode II testing. No increases in mode I toughness were observed. It was, however, found that the aligned interleaves increased the mode II toughness of the composite laminate (from 1.9 ± 0.3 to 2.7 ± 0.2 kJ m⁻²). This increase is demonstrated to be due to a combination of nanofibre bridging and the formation of microcracks in the resin under applied tensile and shear load.