We have used a novel application of in situ high pressure mass spectrometry to monitor the SiH4 + WF6 tungsten chemical vapor deposition process. We observed the production of both SiF4, and SiHF3, in ratios that vary with the SIH4/WF6 inlet flow ratio. The SiF4 and SiHF3 partial pressures were monitored over a tungsten coated wafer at 320 degrees C as the SiH4/WF6 inlet flow ratio was varied from a selective (0.25) to a nonselective (3.0) regime. We observed the SiHF3/SiF4 product ratio to decrease dramatically at reactant ratios associated with the onset of loss of selectivity (1-1.5). The reaction products were sampled by means of a fused silica capillary that was introduced between the reacting tungsten coated wafer and a differentially pumped mass spectrometer. Gas phase reaction and thermodiffusion effects were also observed. These data provide insight concerning loss of selectivity mechanisms, and illustrate the potential of this mass spectrometric technique for real-time in situ process control.