Thermal silicon oxide-to-oxide bonding was investigated at the nanometer level using X-ray reflectivity, transmission electron microscopy, and infrared absorption spectroscopy. The measurements reveal the stages of the closure mechanism, which are different from standard silicon bonding. Upon annealing, interface water pockets are formed, the contents of which are further dissolved into the oxide, demonstrating that the buried thermal oxide-silicon interface acts as a barrier against water reaction with silicon. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3193533] All rights reserved.