There is considerable interest in understanding particle formation in microelectronic fabrication processes since process generated particles are a major source of yield loss in the industry. In this work, particle formation in a plasma enhanced chemical vapor deposition process has been studied using a newly developed instrument—the particle beam mass spectrometer (PBMS)—capable of measuring number densities and size distributions of submicron particles in vacuum environments with pressures>50 mTorr. Experiments have been conducted during the deposition of amorphous silicon, oxide, and nitride films, and particle formation correlated with process parameters such as plasma power and substrate temperature. For the measurements reported, the PBMS has been operated in a downstream monitoring mode, i.e., the PBMS sampled gases from the reactor exhaust during the deposition. Particle formation was observed during the amorphous silicon and oxide runs, but not during the nitride experiments. For the processes in which particle formation was observed, measured particle concentrations decreased with decreasing plasma power and increasing substrate temperature. The measured particle size distributions peaked at ∼0.09 μm for the amorphous silicon, and ∼0.04 μm for the oxide runs. The measured particle diameters compared reasonably well with values obtained from scanning electron microscope (SEM) analysis of particle samples collected on SEM stubs during deposition.