In order to increase our understanding of the chemistry of molybdenum (Mo) in spent oil shale disposal environments, predict fate and behavior, and assess the effectiveness of proposed mitigation techniques, we feel that is imperative to understand the processes controlling Mo aqueous- and solid-phase distribution. Recent research supports the significance of alkaline earth and alkali metal ion pairs with molybdate in aqueous solutions. The objectives of this study were to measure the ion association constants (log Q) for the formation of calcium and magnesium molybdate ion pairs by potentiometric titration at pH 7 and 25{degree}C in NaNO{sub 3} ionic media. The measured values were then used to examine the equilibrium solubility of powellite (CaMoO{sub 4}) in KNO{sub 3} solutions and spent oil shale systems. Powellite solubility data were evaluated using log Q for CaMoO{sub 4}{sup 0} formation, resulting in a calculated log K{sub sp} for powellite dissolution of -7.84. Application of an ion association model that used the molybdate ion pair formation data shows that spent oil shale leachates are undersaturated with respect to powellite. The results of this study show that molybdate ion pair formation in spent oil shale solutions may be significant and that soluble Mo concentrations may not be supported by powellite dissolution. 31 refs., 10 figs., 4 tabs.