Defects generated by 10 eV photons in ultrathin (3-6 nm) SiO2 layers on Si are studied using electron spin resonance and laser photoionization spectroscopy in relation to the observed enhancement of electrical conduction of the oxide. The leakage current is found to include two components: a transient one, related to thermally unstable defect states, and a thermally stable portion. The former current behavior strongly resembles that of the donor interface stales observed in Si/SiO2 structures with thick oxides and is associated with bonding of atomic H in the oxide layer. The stable component correlates with the H-assisted oxide damage resulting in Si-O bond rupture. The exposed similarity between leakage currents observed after irradiation and after electrical stress suggests the hydrogen bonding in these two states to be responsible for the dielectric failure of SiO2 under stress conditions.