One‐dimensional direct simulation Monte Carlo calculations have been carried out on H/H2and H/H2/CO mixtures under operating conditions typical of diffusion‐dominated diamond chemical vapor deposition processes. Mechanisms have been included in the model for the adsorption and recombination of hydrogen atoms on the diamond surface and the dissociation of molecular hydrogen at the interior of the reactor. Hydrogen atom fluxes and recombinative and conductive heat fluxes to the diamond surface are calculated as a function of pressure, gas composition, hydrogen dissociation and surface reaction probabilities, reactor temperature, and distance between the activating source and substrate. The numerical calculations are shown to be in excellent agreement with analytical results in the limiting regimes of free‐streaming particles at low pressures and continuum hydrodynamics at high pressures. ©1996 American Institute of Physics.