The nucleation and morphology of diamond crystals and films synthesized by the use of a combustion flame have been investigated. By operating an oxy-acetylene torch in a fuel-rich mode, diamond crystals and films have been deposited on mechanically abraded molybdenum, on in situ created molybdenum carbide, and on thin diamond-like carbon (DLC) layers synthesized on molybdenum. Scanning electron microscopy, Auger and Raman spectroscopy have been used to characterize the films and crystals. Diamond is found to be uniformly deposited in the region of the substrate that intersects the inner, acetylene-rich region of the flame. The nucleation density, the growth rate, and the morphology of the diamond crystals and films are found to be strongly influenced by the surface condition of the substrate. On mechanically abraded molybdenum, abraded with 600 mesh silicon carbide, and on molybdenum carbide, well-formed cubo-octahedrons of diamond, up to 45 μm in diameter, are formed for deposition times of 90 min. Film formation is seldom observed under these conditions. To enhance nucleation, thin layers of DLC were formed on molybdenum substrates by reducing the O2/C2H2 ratio in the gas mix to ∼ 0.75 for short periods of time under 30 s. This was followed by increasing the O2/C2H2 ratio to conditions that produce diamond (an O2/C2H2 ratio of ∼ 0.9). Under these conditions the nucleation density of diamond was increased by an order of magnitude and the growth rates by about 60%, as compared to diamond deposited on abraded molybdenum and molybdenum carbide. In addition, the morphology of the diamond crystals and films was substantially affected with indications of dendritic growth. The DLC layer is effective in promoting diamond nucleation due to the high surface defect density and the high hydrogen concentration of these films. The combination of surface defects, in the form of dangling bonds, and the evolution of hydrogen from the DLC layer during the diamond deposition process, which is characterized by higher temperatures, result in a high concentration of active surface sites for diamond nucleation. The nucleation density, the distribution on the substrate, and the morphology of diamond crystals and films are not driven by the transport of reactive specie in the flame to the substrate, but rather by nucleation processes, temperature distribution across the surface, and attendant surface phenomena.