Optical transmission of unbacked thin films of thermally grown SiO2films were measured in the vacuum uv. The absorption data indicate a band gap of 8.0±0.2 eV for SiO2, and this value is confirmed by measurements of electron and hole photoconductivity and positive charging thresholds in MOS samples. The observation of hole photoconductivity near 8 eV shows that the uppermost valence levels in SiO2are not nearly atomic, but form a band with sufficient orbital overlap that readily transport holes at room temperature.