The transport properties of 818-eV electrons in SiO2 are studied via the changes induced in the soft-x-ray-induced core-level photoemission peak of the Si 2p substrate by hot-electron transport through the SiO2 overlayers. The observed systematic asymmetric broadening of the substrate line shape with overlayer thickness is shown to be caused by phonon excitation during transport through the overlayer. This effect is strongest at 8 eV and rapidly decreases with increasing electron energy over the range from 8 to 16 eV. These strong dependences on energy and oxide thickness allow the determination of absolute energy-dependent scattering rates for electron-phonon and electron-electron scattering. The analysis is based on the reconstruction of the Si 2p substrate line by means of semiclassical Monte Carlo transport simulations including the electron-phonon interaction and electronic excitation. Acoustic-phonon scattering is found to be the dominant electron-phonon scattering mechanism. This scattering rate decreases slightly with increasing energy. Absolute values in the range from 6×1015 sec-1 (at 8 eV) to 4×1015 sec-1 (at 16 eV) are measured. The reconstruction of the substrate lines requires a deep-inelastic-scattering process, most likely due to electron-hole pair excitation by electron impact. This rate rapidly increases with increasing energy. Values in the range from 2.5×1014 sec-1 (at 8 eV) to 3×1015 sec-1 (at 16 eV) are found. A series of simulations is presented that illustrates the role of various scattering processes in SiO2 at high electron energies. © 1991 The American Physical Society.