Electro-reduction of spent nuclear fuel has the potential to significantly reduce the amount of high level waste from nuclear reactors. Typically, spent uranium and plutonium are recovered via the PUREX process leading to a weapons-grade recovery; however, electro-reduction would allow spent nuclear fuel to be recovered effectively whilst maintaining proliferation resistance. Here, we present predominance diagrams (also known as Littlewood diagrams) for both uranium and plutonium species in molten lithium chloride-potassium chloride eutectic (LKE) at 500 A degrees C and in calcium chloride at 800 A degrees C. All diagrams presented depict regions of stability of various phases at unit activity in equilibrium with their respective dissociated ions. The diagrams thermodynamically define the electrochemical system leading to predictions of reaction conditions necessary to electrochemically separate species. The diagrams have been constructed using a pure thermodynamic route; identifying stable species within the molten salt with an assumption of unit activity for each of the phases. These thermodynamically predicted diagrams have been compared to the limited available experimental data; demonstrating good correlation. The diagrams can also be used to predict regions of stability at activities less than unity and is also demonstrated.