In an investigation of the mechanism responsible for the transport of water through Si02 thin films, we have performed tracer diffusion measurements involving network 0 demonstrating the importance of oxygen exchange between the Si02 network and molecularly dissolved water. We have found that in the presence of water, bound network oxygen diffuses through Si02 as a constituent of molecularly dissolved water. Employing methods common to state-of-the-art semiconductor technology, the central region within a thermal oxide layer grown on silicon was enriched with immobile "O by ion implantation. After heating in atmospheres with different water contents, the extent of 0 diffusion was determined by observing changes in the concentration profile of implanted "O by means of nuclear resonance profiling making use of the 629-keV resonance of the *0{p,a] N reaction. Diffusions conducted in steam at 1 atm (at temperatures as low as 250 C) showed an activation energy of about 17 kcal/mol, which is close to that originally measured for water permeation in Si02. Diffusions in both air and dry nitrogen showed a similar activation energy, with respective pre-exponential factors two and three orders of magnitude below the steam value. Diffusion in low-pressure water vapor showed a clearly linear dependence on gas phase water concentration down to 80 ppm. These results are consistent with a model of water diffusion in Si02 in which the diffusion mechanism is the interstitial transport of dissolved molecular water accompanied by a reversible reaction with silicon-oxygen bonds in tie network.