An isothermal thermochemical cycle to split CO 2 based on nonstoichiometric reduction and oxidation of ceria is demonstrated. Carbon monoxide is produced via an oxygen partial pressure swing by alternating inert sweep gas and CO 2 flows over the ceria. The rates of reduction and oxidation at 1500 °C in a porous ceria particle bed are measured for sweep gas and CO 2 flow rates from 50 to 600 mL min -1 g -1 and analyzed to identify cycle operating conditions (gas flow rates and reduction and oxidation durations) that maximize process efficiency. For a solar reactor assumed to operate at 3000 suns concentration and with 90% of the sensible heat of the gases recovered, the optimal cycle uses 150 mL min -1 g -1 sweep gas and 50 mL min -1 g -1 CO 2 at reduction and oxidation periods of 100 and 155 s, respectively. This cycle is demonstrated in an IR imaging furnace over 102 cycles, yielding a stable average rate of CO production of 0.079 μmol s -1 g -1 and a projected reactor efficiency of 4%. The optimal conditions apply at large scale if the flow rates are scaled in proportion to the ceria mass.