The relationship between the microstructure and mechanical properties of transient liquid phase (TLP) bonds between single crystal CMSX-4 and polycrystalline IN939 and IN738, using wide-gap composite interlayers of Niflex-110 and Niflex-115, is investigated in this paper. A series of interrupted bonding tests were performed to study the bond microstructural development and are correlated with room-temperature mechanical properties. Post-bond microstructural investigations using scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM/EDS), showed the preferential flow of eutectic along the grain boundaries of the Niflex core, rather than the wetting of the faying surfaces, leaving non-bonded regions at the latter. This problem was overcome by increasing the boron content of the Niflex foils, to form additional liquid. Suppression of bondline boride formation was achieved with the wide-gap composite interlayers. The extent of gamma-prime (γ') formation was found to have a greater influence on bond mechanical properties than the formation of second phases in the diffusion zone of the polycrystalline superalloy substrates.