Trapping characteristics of both electrons and holes in pyrogenic oxides grown on silicon with different H2O partial pressures in the oxidizing ambient are studied by using the avalanche injection technique. The electron- and hole-trapping phenomena show opposite dependence on the pyrogenic oxide growth conditions. It is found that the electron-trap density increases with the H2O partial pressure, whereas the hole-trap density decreases. On the basis of the above, and from the results of further experiments on water-diffused oxides, the effect of post-metallization annealing, and infrared absorption, we examine different models of possible defects in silicon-dioxide films grown on Si for a consistent explanation. From the analysis SiH and SiOH centers appear to be the most likely hole and electron traps, respectively, in our pyrogenic oxides.