An isothermal thermochemical cycle to split CO2 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 CO2 flows over the ceria. The rates of reduction and oxidation at 1500 degrees C in a porous ceria particle bed are measured for sweep gas and CO2 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 g(-1) sweep gas and 50 mL min(-1) g(-1) CO2 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 mu mol 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.