Abstract: In the thermochemical water splitting process by Cu—Cl cycle, oxygen gas is produced by a thermolysis process in a three-phase reactor. IN accordance with the teachings herein, a technique is provided to achieve the high challenging thermal requirements of the thermolysis reactor, whereby an optimized heat transfer configuration is used. The technique involves using some of the pre-heated stoichiometric oxygen gas produced from the thermolysis reaction, to transfer heat directly to the slurry of molten CuCl and solid Cu2OCl2 inside the thermolysis reactor. Experiments were performed to examine the volumetric heat transfer coefficient for the direct contact heat transfer between the gas and the slurry. It was found that the thermal scale up analysis of the thermolysis reactor with direct contact heat transfer, is based on the amount of heat carried by the oxygen gas rather than the amount of heat transferred by direct contact heat transfer.