In situ mass spectroscopy is used to sense plasma deposition of silicon and silicon nitride, to analyze gas phase reactant depletion, and the efficiency of silane conversion into the thin film. A double-differentially pumped quadrupole mass spectrometer was used to monitor the SiH4, Si2H6, and HZ effluent from 100% SiH4 and 2% SiH4/He silicon deposition, and SiH4/He/N-2 silicon nitride deposition processes. No significant changes in gas phase nitrogen related species were observed during nitride deposition. However, the Si species show significant process dependence allowing reaction analysis. Disilane species were produced at low powers in the SiH4/He/N-2 process, but no amino-silane species were observed. The silane consumption and silicon incorporation efficiency are shown to depend on gas dilution, residence time, and reactor geometry.