Uranium and vanadium in sandstone-type deposits of the western United States apparently have been transported to their present environment from external sources by low-temperature aqueous solutions. In this paper an attempt is made to interpret the characteristics of aqueous solutions capable of transporting significant quantities of uranium and vanadium through continental sedimentary rocks, and the changes in these characteristics that might result in precipitation of uraninite and other ore minerals in concentrations of ore grade. On the basis of present knowledge, the transportation environment is shown to be that of weakly alkaline, moderately reducing ground water, with an average or larger than average concentration of dissolved carbonate species (H 2CO 3 + HCO 2 -+ C0 2 -"). Precipitation is induced by reduction, probably by carbonaceous material or hydrogen sulfide, or both. Uranium is transported mainly in the form of the highly stable uranyl dicarbonate and tricarbonate complexes. Precipitation results from reduction of hexavalent aqueous uranium species to form uraninite, reduction of tetravalent vanadium to form montroseite, and fixation of uranyl ions by combination with potassium ions and quinquivalent vanadium to form the mineral carnotite.