Possible causes of instability of sulfur-impregnated bodies (porous glass and portland cement) have been investigated. As they are noncrystalline and have a large surface-to-volume ratio, impregnants in small pores are normally in a metastable state unless their free energy is lowered by meniscus effects or interaction with the matrix. Extrusion of sulfur on exposure to water or other adsorbing vapors occurs because of the weak interaction between impregnant and the siliceous surface and a lubricating effect created by the presence of the adsorbate on the surface of the impregnant. Non-isotropic expansion and impregnant - adsorbate interaction are suggested as the main causes of mechanical breakdown. Good interaction with the surface and resistance to flow due to cross-linking render polymethyl methacrylate a stable impregnant.