Abstract When the surface energy is reduced, a material expands exhibiting a swelling phenomenon. This phenomenon can be easily observed when the material has a large surface area. Using a partially leached porous glass, the stress caused by the swelling was measured in various solvents. Mechanical strength of the high silica glass prepared by consolidation of the porous glass was also measured in various liquids. The extent of swelling was found to be well correlated with the mechanical strength reduction. It was suggested from these observations that the environmental effect on the mechanical strength of glass is due to the swelling of the surface.