We investigated water permeation properties into silicon oxide (SiO2) and chemically modified SiO2 films. The water permeability of fluorinated SiO2 (SiOF) is higher than that of SiO2, indicating that SiOF may have fast diffusion paths constructed from SiO-F structures formed by the breaking of Si-O-Si networks by F atoms. It is suggested that the presence of these fast diffusion paths can explain the reason for the high water permeability of various kinds of SiOF and other low-k films.