A quantitative evaluation of the solubility of uraninite (UO2) in aqueous solutions under hydrothermal conditions was made using previously reported thermodynamic data, so as to inquire into the controlling factors for Canadian unconformity-type ore mineralization as observed in the Athabasca uranium field. The results of solubility calculations suggest that uranyl carbonate complexes, such as UO2CO~, UO2(CO3) 2-and UO2(CO3)~-, predominate under relatively oxidizing and slightly acidic-alkaline conditions and that the uranyl chloride complex, UO2 C1 ยง is dominant under acidic conditions. These features are predicted at temperatures up to 200 ~ over reasonable ranges of CO 2 pressure (Pco 2) and salinity. Consequently, the physico-chemical parameters, such as oxygen activity (ao2), and pH are regarded as the most important factors controlling uraninite solubility. Judging from the paragenetic sequences observed in most unconformity-type uranium deposits in the Athabasca district, appreciable decreases in the above variables are postulated to have occurred in the stage of principal uranium deposition. Such changes would be due to fluidmixing phenomenon accompanied by the 'diagenetic-hydrothermal' activity (Hoeve and Quirt 1987).