This paper presents an experimental investigation into the effects of the application of carbon nanotube (CNT) based nanopolymer, and thin film buckypaper, to the interface of stiffened carbon fiber reinforced polymer (CFRP) composite joints. Bonded CFRP composite T- joints, were manufactured with dispersed CNT epoxy nanopolymer mixture, and buckypaper films, applied at the joint interface, and tested under pull-off loading. The presence of the nanomaterial at the interface causes a localized out-of-plane reinforcement, which resists pull-off loads, leading to superior performance compared to composite bonded joints without nano-reinforcements, however, the introduction of substantial voids, in the case of the buckypaper samples, lead to faster structural failure. Digital image correlation (DIC) was used to map the strain contours of the T-joint specimen during testing, which revealed damage initiation and hot-spot zones. Fluorescent optical microscopy of the joint sections was also performed to investigate these hot-spot zones and damage initiation areas, at the mesoscale, to study the possible causal mechanisms of the failure process in the tested composite bonded joints.