Z-contrast imaging and electron energy-loss spectroscopy with a spatial resolution at the atomic scale provide evidence of an atomically abrupt Si-SiO2, interface. Th micrographs revealed no indication for an interface layer of crystalline oxide at this thermally grown interface. Theoretical ab-initio calculations of two different interface structures showed that even in the most ideal interface the local density of states extends into the region of the oxide band gap. The O-K energy-loss near-edge structure was simulated for both interface models. The comparison of theoretical and experimental results of the O-K near-edge structure agreed and showed that states below the conduction band of the oxide are caused by the dimer-like SiO-Si bridges present in all structural models.