This work explored the incorporation of a triblock copolymer in carbon fiber reinforced epoxy polymer composites (CFRPs) to improve their mode-I fracture toughness, G(Ic) (J/m(2)). The triblock copolymer poly (styrene)-block-poly,(butadiene)-block-poly (methylmethacrylate) (SBM) was used to modify the CFRP matrix at 5, 10 and 15 phr concentrations, respectively. CFRPs were manufactured using an in-house sizing tower system, prepregger, vacuum bag and autoclave method. Mode-I fracture toughness testing revealed a 290% increase in G(Ic) by incorporation of the reactive sizing on the fibers and 10 phr SBM in the matrix. Scanning electron microscopy of the SBM modified CFRP fracture surfaces showed that well distributed, sub 100 nm spherical micelles of SBM underwent cavitation and induced void growth and shear yielding toughening mechanisms to absorb fracture energy. It is noteworthy that longitudinal and transverse composite three point flexural testing showed that the SBM modified matrix did not decrease CFRP strength and stiffness up to 10 phr additive. Further, dynamic mechanical analysis revealed that SBM at 10 phr decreased the glass transition temperature (T-g) of CFRPs by only 2.9 degrees C; the Tg was then recovered at 15 phr SBM. Finally, the SBM modified CFRP G(Ic) was compared to the neat matrix G(Ic), at 0, 5, 10 and 15 phr SBM to develop a 'transfer factor' for SBM modified composites. It was found that only 10% of the increased matrix toughness was transferred from the SBM modified epoxy to the CFRPs. The presence of the rigid carbon fibers constrains plastic deformation of the modified epoxy and limits the toughness transfer in the composite. (C) 2017 Elsevier Ltd. All rights reserved.