Only few reaction engineering studies on photocatalysis have been reported, although such studies are necessary for its successful application. The authors studied reaction engineering matters such as the effects of temperature, reactant concentration, catalyst amount, and irradiation intensity on the photocatalytic reaction rate of hydrogen generation from H2S, a waste generated in the fuel oil desulphurization process. CdS/ZnS photocatalyst, which can be activated by solar light, was spread over the bottom of a beaker containing an H2S water solution and irradiated by a Xenon lamp. The photocatalytic hydrogen generation rate was independent of the catalyst amount as far as the photocatalyst particles cover the bottom surface. The reaction rate increased with but less than in proportion to the irradiation intensity, suggesting that a rate limiting step other than photo-activation exists. The activation energy of the reaction, 38 kJ mol(-1), suggested that an elevated reaction temperature would be advantageous. The reaction rate is the first order of the reactant concentration, suggesting that the adsorption of the reactant could be a rate limiting step. The external quantum yield was 19%, comparable to solar cells.