The performance of a 4.4 kW solar receiver/reactor to split carbon dioxide via the isothermal cerium dioxide thermochemical redox cycle is characterized during steady-state operation in a high-flux solar simulator. The solar reactor is the first to implement the isothermal redox cycle. Design innovations for continuous fuel production and gas-phase heat recuperation distinguish it from prior works. During steady-periodic operation at 1750 K, 360 mL min-1 of CO is produced continuously over 45 redox cycles, and up to 95% of the sensible heat of the process gases is recovered. The solar-to-fuel efficiency is 1.64% without consideration of the energy costs of producing nitrogen used as a sweep gas during reduction. With inclusion of the solar energy required to produce N2 via cryogenic separation, the efficiency is 0.72%. On the basis of the thermodynamic limitations of the cycle and the limited opportunity for increasing reactor efficiency beyond 2%, we conclude that the isothermal approach to split CO2 or water via a thermochemical metal oxide redox cycle is not attractive for future development. Future research should leverage the demonstrated advances in reactor design that permit continuous fuel production and recovery of the sensible heat of process gases for alternative cycles such as hybrid isothermal reforming/redox cycles or two-temperature metal redox cycles capable of solid-phase heat recovery.