The diffusion of water into silica glass was assumed to result from the diffusion of molecular water into the glass, and its reaction with the Si-O network, to form SiOH groups. Equations were presented for this diffusion-reaction mechanism, and were compared with experimentally determined diffusion profiles. At temperatures above about 500C, the reaction went to equilibrium while, at lower temperatures, it did not; thus leading to a time-dependence of the concentration of surface-reacted OH groups and of their apparent diffusion coefficient. At higher temperatures, the OH groups were almost immobile, but diffused far enough to meet neighboring OH groups; thus leading to a bi- molecular reverse reaction. At lower temperatures, only the OH pairs reacted; thus leading to a first-order reaction. When water which was tagged with 18O was diffused into silica, the 18O exchanged with 16O in the Si-O network of the glass. This process was also controlled by the rate of diffusion of molecular water into the glass, and by the rate of 18O-16O exchange. The present mechanism provided a consistent description of the diffusion of water in silica at temperatures of between 160 and 1200C.

R.H.Doremus: Journal of Materials Research, 1995, 10[9], 2379-89