Optical transmission and photoconductivity spectra (7-14 eV) and the field dependence of photoconductivity are presented for thermally grown (amorphous) SiO2 films. It is argued that the clearest experimental determination of the band gap in SiO2 can be obtained from the field dependence of photoconductivity and its similarity to internal photoemission; a band gap of 9.3 eV for amorphous SiO2 is deduced from the data. In view of this result some recent experimental band-gap determinations are criticized and the literature on this subject is examined. The transmission experiments were performed on thin thermally grown (amorphous) SiO2 films (450-5000) produced by etching off the silicon substrate and using a fabrication method which is described in detail. The photoconductivity measurements were performed on Al-SiO2-Si structures (SiO2: 600-3500). Diode experiments for detecting photoluminescence and possibly excitons are also described. The upper limit on photoluminescence yield was determined as ∼10-4. © 1979 The American Physical Society.