The surface of SnO2 was modified by fluoroalkyltrichlorosilanes (FAS) with varying molecular length in gas phase. Owing to the large infrared reflectivity and the surface asperity of the underlying SnO2 film, the signal intensities in Fourier-transformed infrared attenuated total reflection (FT-IR ATR) spectra from the outermost FAS-monolayer spectra greatly increased compared to those from the layer formed on glass substrates. The molecular orientation normal to the substrate plane in the FAS-layer was enhanced with an increase in the molecular size. Both the FT-IRATR and O-1s-XPS spectra suggested that FAS molecules react with the isolated Sn-OH groups to form chemical bonds on the surface of SnO2, and produced intermolecular networks.