A uranium oxide reduction process under development at Argonne National Laboratory utilizes a Li/LiCl-based system at 650degreesC. When UO2 is introduced to this system, the lithium metal reduces UO2 to metallic uranium and generates Li2O. A ThO2 solid electrolyte was selected and tested as an oxide-ion sensor to monitor the concentration of Li2O in the molten LiCl during the uranium reduction process. Experimental results indicated that a ThO2 probe doped with 7.5 wt % Y2O3 was effective at real-time monitoring of the oxide ion resulting from incremental additions of Li2O to the Li/LiCl-based system. The experimental results also indicated that the activity and activity coefficient of Li2O in molten LiCl at 650degreesC are consistent with Raoult's law.