The charging of buried oxide (BOX) of bonded and etched-back silicon-on-insulator (BESOI) structures caused by electrical stress was studied. The top SI layer was removed and the structures with bare oxide were immersed in H2O before evaporation of aluminum. This treatment results in an ion-type charge instability effect observed at room temperature. If the top Si layer was removed by dry etching or not removed at all, then the structures do not exhibit a comparable effect even at 250 degrees C; neither do wet-treated nonbonded thermal SiO2 layers. Replacement of H2O by D2O delays the ionic charge buildup, suggesting an isotopic effect. It is suggested that the high-temperature postbonding anneal results in structural change in the BOX layer which promotes the transport of hydrogen-related charged species.