The deposition process of W from WF6 onto a-Si:H:F film was studied by polarization modulation IR spectroscopy combined with quadruple mass spectrometry. The IR spectrum of the a-Si:H:F film formed by the spontaneous chemical deposition method (deposition temperature 373 K) showed that such fluorinated and hydrogenated Si species as -SiF2, -SiF2H and -SiH2 were incorporated in the film. The lowest temperature at which W was deposited was 403 K. The intensity reduction of the IR bands due to the hydrogenated and fluorinated Si species as well as the evolutionary behaviour of the reaction products SiF4 and H2 depended upon the reaction temperature. At temperature between 403 K and 423 K, the IR band intensity of the -SiH2 species remained unchanged in early stages of WF6 exposure, while the bands due to -SiF2 and -SiF2H species decreased linearly with exposure time. No evolution of H2 was observed as long as the -SiH2 species remained unchanged, through SiF4 evolved into the gas phase just after the beginning of the exposure. These results suggest that WF6 reacts preferentially with the fluorinated silicon species in the films.