Mass spectrometry has been exploited for rapid real‐time sensing of both reactant and product species in single‐wafer rapid thermal chemical vapor deposition (RTCVD) of polycrystalline Si from SiH4. Active mass spectrometric sampling at pressures to 5 Torr is achieved using two‐stage differential pumping of a sampling aperture in the exhaust stream, leading to response times as short as ∼3 sec to concentration and pressure changes in the reactor during a process carried out in ∼30 sec. In addition to reactant species, gaseous reaction byproducts have been identified and differentiated from cracking fragments of the reactant through relative intensities of mass fragments as a function of wafer temperature (i.e., reaction rate). For RTCVD of poly‐Si from SiH4, carried out in the range 450–800 °C at 5 Torr in 10% SiH4/Ar, mass spectra reveal not only the time dependence of reactant (monitored by SiH2+, 30 amu), but also—at higher temperatures—reactant depletion and product generation (from H2+, 2 amu). These results demonstrate a basis for using mass spectrometry in real‐time process diagnostics and control.