The suitability of clean coal technology by-products as impervious liners in place of commonly used clay or geomembranes is investigated. A laboratory program to characterize the permeability of stabilized flue gas desulfurization (FGD) material is presented. Several mixes were prepared and tested. The laboratory samples exhibit low permeabilities and high strengths necessary for liner applications. The permeability values measured in the laboratory for some samples are much lower than the 1 x 10(-7) cm s(-1) value required by EPA for constructing liners for waste containment facilities.