Diamond crystallites have been deposited in the temperature range 813-1173 K on polycrystalline sintered tungsten substrates using the Hot Filament Chemical Vapour Deposition (HFCVD) technique with the aim of attaining information specifically on the nucleation behaviour of the diamond phase. The nucleation density measured on scratched specimens has been found to decrease on increasing the substrate temperature. The reverse trend has been observed for the formation of the tungsten carbides WC and W2C, suggesting a competition between carburization and formation of the stable nucleus. Annealing of scratched samples at 1473 K in dihydrogen has a dramatic effect on the nucleation density which falls by about 3-4 orders of magnitude and becomes comparable with that observed on unscratched specimens. The role of linear lattice defects in determining the nucleation behaviour is also discussed. © 1992.