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Diffusion of water into silica glass was measured in the temperature range of 200" to 750°C by treating the glass in air containing a constant water vapor pressure aind analyzing the concentration profile using a Fourier transform infrared spectrometer. In the short-time diffusion heat treatments, the surface concentration was lower and the apparent diffusion coefficient was higher than the corresponding steady-state values. The temperature dependence of the steady-state diffusion coefficient showed two different activation energies. Above -550°C the diffusion coefficients were similar to the published results with an activation energy of -80 kJ/mol, while below -550°C, the diffusion coefficient was higher than the value obtained by extrapolation from lhigher temperatures, and the activation energy was -40 kJ/mol. Correspondingly, the water solubility-temperature relation showed a sudden change at around the same temperature: at temperatures above this temperature the solubility increased with decreasing temperature, while at lower temperatures the trend was reversed. It was suggested that this observed peculiarity was caused by the initial nonequilibrium reaction between water and S O z glass and a change in enthalpy of the glass-water reaction. [