A quantitative model of the deposition of silicon films by hot-wire chemical vapor deposition (HWCVD) is presented, and its predictions are compared with experiments. The model equations describe a vacuum reactor in which silane cracks over a series of heated tantalum wires, reacts further in the gas phase, and deposits in the form of silicon films on glass substrates. The model considers gas motion in the reactor and incorporates surface pyrolysis reactions on the hot wire, gas-phase reactions, and film-growth reactions on the substrate. The model predictions of silane conversion and silicon film growth rate are in good agreement with the experimental results over the range of conditions studied. This study shows that a critical ratio of atomic hydrogen flux relative to the total flux of growth precursors is required at the film surface for transition from amorphous to polycrystalline silicon films. The flux ratio of hydrogen radical to growth precursors is controlled by the pressure, filament temperature, and silane flow rate. For the conditions investigated, a flux ratio greater than 15 leads to the deposition of polycrystalline silicon films.