Monte Carlo simulations of hot-wire chemical vapor deposition (HWCVD) gas ambients indicate different flux ratios (SiH3/Si and H/SiHx) under conditions for amorphous or polycrystalline silicon growth. Gas-phase reactions of Si with ambient SiH4 studied using abinitio methods reveals that collisional stabilization of the adduct (H3SiSiH) is unlikely under typical HWCVD growth pressures, but an energetically favorable, low-pressure pathway has been identified that leads to the formation of Si2H2 and H2. Threshold ionization mass spectrometry has revealed significant quantities of the radical SiH2 at HWCVD growth pressures, indicative of heterogeneous pyrolysis. Mass spectrometry at low pressures suggests that incident silane dissociatively adsorbs at the wire and undergoes sequential H elimination to produce subhydrides. Disilicon species were not detected in significant quantities at HWCVD growth pressures. Finally, hot wire operation in a pure H2 ambient yields SiH4 as the dominant etching product from the silicon-coated walls of the growth chamber.