Real-time, in situ chemical sensing has been applied to achieve reaction metrology and advanced process control in a low pressure tungsten chemical vapor deposition process based on WF6 and SiH4 reactants (silane reduction process). Using mass spectrometry as the sensor to detect both product generation (H-2) and reactant depletion (SiH4) at wafer temperature of 200-250degreesC, these signals provided a direct real-time measurement of deposited film thickness with an uncertainty less than 2%, and this thickness metrology signal was employed to achieve real-time process end point control. When reactant conversion rates are sufficient (similar to20% in. this case) as often occurs in manufacturing processes, the thickness metrology (1.0%-1.5%) and control (similar to1.5%-2.0%) accuracies are in the regime needed for meaningful application of advanced process control. Since the in situ sensor delivers a metrology signal in real time, real-time process control is achieved, enabling compensation for random process disturbances during an individual process cycle as well as for systematic wafer-to-wafer process drifts. These results are promising for manufacturing from the standpoints of metrology accuracy and application in real-time control.