Experiments employing molecular beams of atomic hydrogen and a hydrocarbon precursor were carried out in an attempt to deposit diamond films at the pressure of 10\sp−4 torr. No diamond films were deposited using methane, methyl iodide or di-tert-butyl peroxide as carbon sources. Only amorphous carbon films were deposited under some circumstances. By using a carbon-13 labeling technique, it was found that chloromethanes contribute to diamond growth via a new growth precursor(s) in the form of chlorocarbon radicals in addition to the methyl radical pathway in a hot filament reactor. On the other hand, fluoromethanes, bromomethane and iodomethane yield diamond through the methyl radical mechanism. It was argued that the drastically different behaviors among halocarbons in diamond CVD systems are attributable to the differences in their thermodynamic properties and kinetic parameters of the reactions of halocarbons with atomic hydrogen. It was demonstrated that low temperature deposition of diamond films can be achieved by either using chlorocarbons as carbon sources or adding HCl to a methane/hydrogen system. The major function of HCl under diamond CVD conditions is to generate chlorine atoms. These chlorine atoms in turn activate the hydrogenated diamond surface via a more efficient pathway, i.e., chlorine abstraction reaction of surface-adsorbed hydrogen atoms at low temperatures. As a result, diamond films were deposited by using methyl chloride at substrate temperatures as low as 300\sp∘C. Studies of the dependence of the growth rate on the substrate temperature revealed the existence of two growth regions, i.e., the transport/diffusion-limited regime at high temperatures and the surface-controlled regime at low temperatures. The transition temperature was found to be around 730\sp∘C and 570\sp∘C for the CH\sb4/H\sb2 and CCl\sb4/H\sb2 systems respectively. It was also found that adding HCl to the CH\sb4/H\sb2 deposition system yielded an increase in growth rates of diamond films at low temperatures. This phenomenon can be qualitatively explained by the Langmuir adsorption isotherm model with a modification by incorporating the Eley-Rideal mechanism for hydrogen abstraction.