Gas-phase diagnostics in the catalytic chemical vapor deposition processes of the SiH4/NH3/H-2 system were carried out to examine the effect of H-2 dilution. The decomposition efficiency of NH3 showed a sharp decrease with the introduction of a small amount of SiH4, but this decrease was recovered by the addition of H-2. When the NH3 pressure was low. On the other hand, at higher NH3 pressures, the decomposition efficiency showed a minor dependence on the H-2 partial pressure. The addition of SiH4 to the NH3 system decreases the H-atom density by one order of magnitude, but this decrease is also recovered by H-2 addition. H atoms produced from H-2 must re-activate the catalyzer surfaces poisoned by SiH4 when the NH3 pressure is low. (c) 2005 Elsevier B.V. All rights reserved.