Diamond deposition has been carried out on commercial purity (CP) titanium and Ti-6Al-4V substrates, using hot filament chemical vapour deposition. interfacial reactions have been studied in derail using scanning electron microscopy, X-ray photoelectron spectroscopy, optical microscopy and X-ray diffraction. Hydrogen suppresses diamond nucleation by etching the diamond debris left on the substrate surface after polishing. Hydride precipitation was observed throughout the substrate. This caused embrittlement. TiC formation on the substrate surface started after 5 min. In the Ti-6Al-4V alloy and in the CP titanium, TIC formation occurred preferentially on the P-phase and on grain boundaries respectively. The growth of TiC competes with diamond formation for the available carbon. Relatively low temperatures inhibit TiC formation more than diamond formation, allowing the diamond nuclei to grow faster in the early stages. The formation of a thick TiC layer also appears to promote interfacial debonding and spallation of the diamond film. Prior bombardment with carbon ions appears to be beneficial as a result of allowing TiC growth to occur without removing as much atmospheric carbon, so that the diamond film can form in more intimate contact with the substrate.