By measuring the curvature of thin disks of vitreous silica that have been penetrated by water from one side only, we determined the volume expansion of the silica and the effect of this volume expansion on its strength. We found that the water-strengthening process depended on crack-size, temperature, and the amount of swelling of the silica. We also evaluated the diffusivity of water in vitreous silica, using the swelling stresses as the diffusion metric. Diffusivity values, so obtained, are close to the accepted values for the diffusion of water in vitreous silica, as is the activation energy for the diffusion process. Our data suggest that swelling and the consequent bending of the disks is caused by silanol group formation in the silica structure; molecular water plays little role in the swelling process.