Potential-pH diagrams are reported for uranium and plutonium in water containing carbonate, chloride, fluoride, phosphate, sodium, and sulfate ions at 25, 100, and 200°C Complexation was considered in deriving the diagrams and the potential-pH relationships were calculated for various total dissolved ion concentrations. The diagrams are used to analyze the general features of the thermodynamic behavior of uranium and plutonium in geological systems and to help identify processes that might be expected to occur in the vicinity of a deep underground vault for nuclear fuel waste.