Interfacial reaction between titanium matrix and reinforcement plays a crucial role in determining the mechanical properties of titanium metal matrix composite materials. In order to improve the mechanical properties of composite materials, it is essential to understand the thermodynamics and kinetics of such interfacial reactions. Ti-6Al-4V foils and C/TiB_x-coated SiC fibres were used to fabricate composite materials by diffusion bonding. The interface formed after annealing at different temperatures has been characterized mainly by scanning and transmission electron microscopies to establish the reaction kinetics between the TiB_x coating and Ti matrix. It is found that the major reaction product is TiB needles, although a TiB₂ layer is also present as a transition phase during the initial stage of the reaction. Experimental results indicate that, at a temperature between 870 and 970°C, the growth rate of TiB needles along the needle direction is more than six times of that of the TiB₂ layer. After a detailed analysis of the crystal structures and the growth morphologies of both TiB and TiB₂, the diffusion mechanisms for B atoms in TiB and TiB₂ have been identified as vacancy diffusion. However, the low activation energy path for B diffusion in TiB is in the [0 1 0]_TiB direction, effectively one-dimensional, while that in TiB₂ is along 〈1 1 0 0〉_TiB2 directions, which form a two-dimensional network. In addition, it is found that the estimated diffusion coefficient for B in TiB along the needle direction is about 45 times larger than that in TiB₂, although the activation energies for B diffusion in both TiB and TiB₂ are effectively the same, being 187–190 kJ mol⁻¹.