The quality of the bond produced after mating oxidized and/or unoxidized silicon wafers has been studied using acoustic microscopy, infrared transmission thermographs, and transmission electron microscopy. The acoustic microscopy revealed that a significant number of unbonded regions (gaps) remain at the bond interface after bonding in oxygen, nitrogen, or high vacuum, and then annealing. These gaps could be virtually eliminated by a subsequent hyperbaric annealing step. The thermal imaging was found to have insufficient resolution to give a detailed picture of the bond quality. Transmission electron microscopy showed that an excellent bond could be produced when bonding clean silicon wafers, with only very small oxide or void bubbles present at the interface. Bonding two oxidized wafers resulted in a buried oxide layer with no detectable bond line. Mating an oxidized wafer to an unoxidized wafer produced a bonded silicon/oxide interface which was nearly indistiguishable from the wafer/thermal oxide interface.