The adsorption of vapor on the internal surface of a sandstone reduces its frame moduli. A theoretical model has been developed which predicts the weakening as a function of fluid chemistry and effective pressure. The model is based on well established contact theory and surface chemistry. Frame moduli are directly proportional to the stiffness of the grain contacts. The contact stiffness iis dependent on surface energy. We show how the adsorption of various chemicals onto quartz surfaces lowers the surface energy, thereby weakening the contact stiffness. The predicted variation of frame moduli with the partial pressure and chemical species is qualitatively consistent with recent experimental results. The calculations suggests that the chemical effect is only significant at low effective pressures. At high pressures, the elastic loading dominated the contribution due to surface energy.