In situ infrared absorption spectroscopy is used to monitor the delivery of condensible gases in chemical vapor deposition (CVD) systems. A Fourier transform infrared (FTIR) spectrometer has been configured to enable simultaneous measurements of the concentrations of multiple gases which are critical to the performance of CVD processes. In the deposition of silicon dioxide (SiO2) films using tetraethylorthosilicate (TEOS) and ozone (O3), IR measurements of the concentration of gas-phase TEOS closely track the deposition rate of SiO2. In doped oxide deposition processes, the percentage of phosphorous in the film is predicted from infrared measurements of the concentration of the dopant precursor gas [trimethylphosphene (TMP)]. These capabilities provide improved control of the delivery of condensible gases in CVD systems without the use of test wafers, and they enable the detection of failures in the gas delivery system in real time.