AbstractHydrogen production by solar thermochemical process uses concentrated solar radiation as its energy source. Various thermochemical processes operating at technically manageable temperatures which are a solar thermochemical two-step water splitting and solar gasification of carbonaceousmaterial have been extensivelystudied and demonstrated by researchers around the world. These processes arecapable of converting high-temperature heat from concentrated solar radiation into clean hydrogen from water.In this study, in order to control a flowability (fluidization state) of bed materials in a fluidized bed reactor for thermochemical processes (two-step water splitting cycle and gasification of coal coke),firstly, a basic relationship between pressure drop of inlet gas and gas flow rate was experimentally examined using bed materials with different particle sizes by a small-scale quartz reactor at ambient pressure and temperature. Secondly, the CeO2 particles having the size determined by above-describedflowability test were tested using a windowed fluidized bed reactor prototype. The fluidized bed of CeO2 particles was exposed to a concentrated Xe light by sun-simulator with an input power of about 5 kWth for the T-R step in order to release oxygen. The production rate and productivity of oxygen and the reactivity of CeO2 particles were examined in this paper.