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This work reports an experimental electron-energy-loss spectroscopy study carried out on a model thermal Si-SiO2 interface. Valence-loss spectra and core-loss spectra (Si L2,3 and O K edges) were recorded across the interface in line-spectrum mode with a high spatial resolution in a field emission gun scanning transmission electron microscope. From the analysis of the line spectra and on the basis of high-resolution electron microscopy and high-angle annular dark-field experiments, it is concluded that the interface is not sharp but extends over about three atomic planes consisting of Si and O atoms arranged in a structure evolving between crystalline SiO and SiO2 before growing as an amorphous SiO2 layer. In addition, from the analysis of the valence-loss spectra in terms of energy-loss function or dielectric function ε, we show that valence-electron-energyloss spectroscopy could be a relevant alternative method for determining the electron properties, for example the bandgap, and the dielectric constant of dielectric gates on a nanometre scale. [ABSTRACT FROM AUTHOR]