Gas-phase diagnostics in the catalytic chemical vapor deposition processes of the SiH 4 /NH 3 /H 2 system were carried out to examine the effect of H 2 dilution. The decomposition efficiency of NH 3 showed a sharp decrease with the introduction of a small amount of SiH 4 , but this decrease was recovered by the addition of H 2 when the NH 3 pressure was low. On the other hand, at higher NH 3 pressures, the decomposition efficiency showed a minor dependence on the H 2 partial pressure. The addition of SiH 4 to the NH 3 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 SiH 4 when the NH 3 pressure is low.