The kinetic aspect of a highly endothermic reaction system for solar thermochemical energy conversion was studied using a Xe lamp. The Xe light beam with an energy intensity of 34 W/cm(2) was irradiated onto carbon in a flow of CO2. The maximum rate of CO evolution was attained at 1700 s of irradiation, and the solar/chemical energy conversion efficiency was ca. 5%. A novel solar thermochemical energy conversion model based on an energy distributing process was proposed to evaluate the chemical energy conversion efficiency. In the model proposed, the chemical energy conversion efficiency, ak, and efficiency of energy absorption, epsilon, were defined. ak varies with the proceeding reaction, since the reaction conditions at each point of the reactor changed with consumption of active carbon by the Boudouard reaction. In the Boudouard reaction, the maximum chemical energy conversion efficiency was estimated to be 16%.