A comprehensive understanding of the molecular process of thin films growing under the chemical vapor deposition (CVD) conditions is quite of importance from a technological point of view. In-situ molecular vibrational study of the dynamic behavior of surface species under such conditions should give us an important clue to the understanding of the process and, further, a useful guide in developing new thin film devices. However, there exist few techniques available for the in-situ observation of the process. The polarization modulation IR spectroscopy (PM-IR), which is based on the difference in absorption intensity of surface species upon the change in the polarization state of the incident IR light, is a powerful tool for detecting signals from surface species over wide ranges of the substrate temperature and gaseous pressure. We have been successful in applying this method to observing in-situ the photo-CVD process of the Si-related thin films as well as the reaction of the films with the metal-organic and the etchant molecules. In the present talk, we will mention (1) the basis of the PM-IR method, the results obtained through the in-situ IR observation of (2) the growing process of the a-Si:H and the a-SiN@sub x@ thin films, and (3) the reaction of the a-Si:H/dimethylaluminum hydride ((CH@sub 3@)@sub 2@AlH) and the a-SiN@sub x@/F@sub 2@. The results suggest that the hydrogen-rich and the Si-rich layers are present on the growing surface of the a-Si:H and the a-SiN@sub x@ film, respectively, illustrating the feasibility of the PM-IR method to the in-situ molecular vibrational study of the chemical process of the CVD thin films.