Quadrupole mass spectrometry has been used to monitor reactant and product partial pressures in a selective W chemical vapor deposition process. A 4/1H(2)/WF6 molar reactant ratio was used to produce W films on Si wafers, at 67 Pa (0.5 Torr) total pressure, and for wafer temperatures around 400 degrees C. A relatively fast response time (similar to 4 s) sensor system sampled gas directly from a commercial Ulvac ERA-1000 reactor in order to minimize the effect of wall reactions. The signal from the volatile HF product, integrated over the deposition cycle, and corrected fur contributions from reactions in the ion-source region of the quadrupole and for sensor drifts, was found to vary linearly with the weight of the W film deposited, to within an uncertainty of similar to 7%. This provides the basis for real-time, noninvasive thickness metrology to drive process control. Depiction of both H-2 and WF6 reactants was observed. The time integral of the H-2 reactant depletion was also linearly related to the film weight, though the data exhibited a somewhat larger scatter due to the low conversion efficiency of the process. In addition, volatile SiF4 and SiHF3 products of the initial rapid W nucleation reaction on the Si surface were clearly observed, indicating that initial surface conditions may bt: monitored in real time under selective growth conditions.