It has been postulated that the molecular water, H2O, is mobile but the hydroxyl water, OH, is immobile in silica glass. Further, it has been theorized that H2O and OH are held in a strict equilibrium. This study investigates the behavior of H2O in the oxidation of silicon: It was found that the strict equilibrium was not held, and fast-moving H2O was found to cause silicon oxidation. It is suggested that the diffusion of molecular water, which can diffuse faster than oxygen and has not been directly observed in a normal water diffusion study in silica glass, is responsible for the faster oxidation of silicon in wet oxidation than in dry (oxygen) oxidation. Some indirect evidence for such fast moving molecular water is presented. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3560037] All rights reserved.