Nanocrystalline films of delta -MoC1-x have been deposited from a MoCl5/H-2/C2H4 mixture at 800 degreesC and a total pressure of 1.7 torr. The average grain size, free carbon content and the deposition rate were affected by different experimental parameters such as vapour composition and substrate position in the reactor. One of the most important parameters was the C2H4 concentration, which at higher concentrations reduces the grain size and the deposition rate. The results suggest that the nanocrystallinity was not caused by a high supersaturation. A more likely explanation is that the observed growth behaviour is caused by strongly adsorbed species from decomposing C2H4 which reduce the surface diffusion lengths during film growth.