In recent decades, fibre-reinforced polymer composite structures have seen increased aerospace applications. A typical composite structural element widely used in primary load-bearing aircraft components is the T-joint, which is formed when two orthogonal laminates are joined in a T-shaped arrangement to enable out-of-plane load transfer. Under tensile loading, the structure tends to fail by the separation of the horizontal and vertical laminates at the joining interface. The incorporation of an interleaf, i.e. a thin layer of material with good interfacial bonding properties, may prevent or delay the failure event. A potential interleaf material is the carbon nanotube (CNT). In the literature, its addition as a reinforcing phase in interleaves has been shown to improve fracture toughness in standard coupon tests, but the technique has yet to be tested on a structural element level. This thesis research investigated the effect of CNT interleaves on the mechanical behaviour of composite T-joints. Manufacturing processes were developed to produce test samples consistent in quality. T-joint samples with CNT-modified interleaves were manufactured alongside neat samples with zero CNT content. The joints were formed using custom-built tools and co-cured in an autoclave. Tensile tests were carried out and the T-joints were loaded until failure. Load-displacement data were acquired and further supported by results from a Digital Image Correlation (DIC) system. Data analysis suggested that CNT interleaves modified the mechanical behaviour of T-joints by delaying the failure of the structure. While their stiffness and ultimate strength were largely unaltered, the CNT-modified T-joints were able to undergo higher displacements and absorb higher strain energy before failure compared to the neat samples. The delay of failure was enabled by extensive crack development within CNT interleaves preceding the final rupture. In comparison, the neat samples failed abruptly after very limited damage evolution. Observation of the crack surfaces revealed evidence of tortuous crack paths through CNT interleaves. The finding suggests that CNT interleaves are capable of improving the failure resistance of the T-joint structure in a targeted fashion.