Structural and electrical properties of thermal SiO2layers on Si subjected to wafer bonding were studied. Regions with enhanced etching rate in HF‐solutions were found to appear after the bonding and a subsequent etchback of one of the wafers. The bonding procedure also leads to enhancement of interface state generation and accumulation of oxide trapped charge during injection of electrons or holes into the SiO2layer. The annealing behavior and capture cross section of these defects are close to those of centers generated when hydrogen is incorporated in the oxide network. Enrichment of the oxide by hydrogen during the bonding and postbonding anneal is suggested to be responsible for the degradation of the thermal oxide. ©1995 American Institute of Physics.