The bending strength of glass slides, which had the dimensions 38×6×0.16 mm, was measured at different relative pressures of water vapor. The adsorption isotherms of water vapor on powdered glass, which had the same composition as the slides, were measured in a separate experiment. The reduction of the surface free energy as function of the vapor pressure was calculated from the isotherms. The observed variation of the strength was compared with the strength variation calculated from the reduction of the surface free energy. The Griffith equation for brittle fracture was used to relate strength and surface free energy. Two cases were considered: (1) the specimens were weakened by the reduction of the surface free energy and (2) the specimens were weakened by the reduction of the surface free energy and by a deepening of the surface cracks. Both interpretations agreed with the experimental results because, by using the Griffith equation, only the lower limits for the strength of the specimens could be calculated.It was found that the observed strength did not decrease steadily with increasing vapor pressure, but had a secondary maximum at a relative pressure of about 0.2. The maximum reduction of the strength corresponding to complete wetting with water was already reached at a relative pressure of about 0.5. The ratio of the strength in saturated water vapor to the strength in vacuum was found to be independent of the surface damage of the specimens.