A literature review  of the leaching behavior of inorganic constituents contained in fossil fuel wastes indicated that most of the available information deals primarily with (1) determination of the elemental composition of different wastes, (2) examination of the physical characteristics of waste solids, and (3) empirical leaching studies involving different solutions and procedures (e.g., water, acids, extraction procedure, toxicity characteristic leaching procedure). A comprehensive mechanistic approach and data are needed to predict accurately the composition of pore waters from fossil fuel wastes. An approach that relates the aqueous concentrations in leachates to solubility-controlling or adsorption-controlling solid phases- in the wastes is described. The behavior of calcium is used as an example to show the relevance of existing data to predicting leachate composition and to identify the type of data needed. The application of this approach to pore waters from flue gas desulfurization sludge shows that Ca concentrations can be accurately predicted from the nature of the Ca solids present and the thermo-chemical data descriptive of precipitation/dissolution and complexation reactions.