Using CH4 and CF4 gases, fluorinated amorphous carbon films were deposited by inductively coupled plasmas at various gas flow ratio R, R=[CH4]/([CH4]+[CF4]). The optical properties and structural evolution of the films were characterized by ultraviolet-visible spectroscopy, Fourier transform infrared transmission spectroscopy and the X-ray photoemission spectroscopic spectra respectively. Hydrogen and fluorine exert influence on the optical band gap in a similar way by forming sigma bonds with carbon and decreasing the number of pi bonds. The variation of hydrogen and fluorine atomic densities in the films are main factors that cause firstly decrease and then increase of the optic band gap with increasing R. (c) 2005 Elsevier B.V All rights reserved.