We have directly observed cathodoluminescence (CL) in ultrahigh vacuum over a broad spectral range (0.7–4.0 eV) from ultrathin 5 nm layers of remote plasma enhanced chemical vapor deposition grown a-SiO2:H deposited on silicon substrates. In the infrared regime, luminescence is observed at 0.8 eV, consistent with the presence in the as-deposited film of Si dangling bond localized states, as well as at 1.1 eV due to band edge emission. In the optical regime, three peaks (1.9, 2.7, and 3.4 eV) are observed showing evidence for band tail state emission from an amorphous silicon–oxygen bonded suboxide region in the film, with smaller contributions from either substrate related c-Si or defect containing, stoichiometric SiO2. CL spectra with a range of beam voltages indicate that the stoichiometric SiO2 is very close to the free surface of the film, possibly due to oxidation of the air-exposed wafer or due to a nonuniformity in the film. When the films are annealed in situ in stages up to 500 °C, we observe no change in the shape of the a-SiOx:H peak at 1.9 eV, showing the stability of this suboxide to such temperatures. These observations are consistent with CL measurements of thicker films of a-SiO2:H and a-SiOx, and demonstrate the utility of CL spectroscopy for the study of ultrathin dielectric studies. © 1998 American Vacuum Society.