This work explores the effect of core-shell rubber (CSR) addition on the cure reaction and resulting mechanical and thermal properties of a highly cross-linked network made from 4,4 '- tetraglycidyl diamino diphenyl methane and 3,3 '-diamino diphenyl sulphone. Up to 20 wt% of a CSR rubber, composed of a soft polybutadiene core and a hard poly methyl methacrylate (PMMA) shell was dispersed in the epoxy/amine and after cure was found to increase the glass transition temperature and thermal degradation temperatures modestly, while reducing the char yield. Flexural properties also reduced with CSR addition, although fracture toughness (K-IC) was increased by 45% at 20 wt% of CSR. Furthermore, the fracture strain energy release rate (G(IC)) of the corresponding carbon fibre composite at the same CSR concentration, increased by almost 115%. Acoustic emission spectroscopy is introduced as a tool for investigating crack propagation during mode I loading and identifying any differences in the failure mechanism between the unmodified and the 20 wt% CSR-modified carbon fibre composites.