Changes of an organic spin-on-glass (SOG) film surface on liquid-phase deposition of fluorinated silicon oxide film in the fabrication of thin-film optical waveguides were studied. Refractive indices for the organic SOC films were decreased uniformly by approximately 1% after treatment with hydrofluosilicic acid (H2SiF6) aqueous solution. Reflectance evaluated at a wavelength of 632.8 nm fur the organic SOG films was increased by 1% after H2SiF6 treatment. No clear change of silicon, carbon and oxygen peaks was observed in the Anger electron spectra after H2SiF6 treatment. Three typical fluorine (F) peaks were observed at the surfaces of organic SOG films after H2SiF6 treatment. The F peaks disappeared after in situ argon ion etching for 10 s. It was clarified that H2SiF6 treatment does not affect the bond structure of Si-CH3 in the organic SOG films. A very weak absorption peak of the Si-F bond was observed in Fourier transform infrared spectra after H2SiF6 treatment. It was speculated that changes of optical properties were due to the adsorption of Si(OH)(4-x)F-6 species to the organic SOG film surface.