Leachate may be generated from piles of oil shale solid waste as a result of drainage of waters emplaced with the solid upon disposal and from net infiltration of incident precipitation. The rate and quantity of leachate that may be generated depends heavily on the capillary hydraulic properties of the waste material. Combusted or decarbonized oil shale waste has been found to be highly reactive. Hydration has a marked effect on the quantity of water available for flow and upon the hydraulic properties affecting the flow. The paper presents data which show that the quantity of chemically-bound water is a function of the water available. Equilibrium partitioning of water into the mobile and chemically-bound states is quantified by an isotherm approach. The partitioning isotherm is used to develop an approximate method for calculating the rate of penetration of mobile waters through reactive oil shale solid waste. Example calculations are used to illustrate the magnitude of retardation of flow caused by the chemical reactions. Hydration and associated production of gas can also cause misleading behavior in laboratory permeability tests. Laboratory experience with chemically-active oil shale residues is described.