In-situ analysis of low temperature Plasma Enhanced Chemical Vapor Deposition (PECVD) SiO2 films deposited on HgCdTe, Silicon, and Aluminum substrates was performed by double modulation Fourier Transform Infrared Reflection Absorption spectroscopy (FT-IRRAS). The sensitivity and selectivity of this technique are sufficient for an in-situ assessment of the film quality and reaction conditions at any stage of film growth. An oblique angle of incidence of ca. 557deg; was chosen to yield maximum sensitivity for the 1260 cm−1 LO mode of SiO2 on Si. The peak frequency and shape of the LO mode absorption band varied with the quality of the SiO2 films. This diagnostic technique can be applied readily to in-situ analysis of dielectric thin films formed under a variety of reaction conditions as long as the gaseous ambient is partially transmissive to the IR radiation.