. We are developing a new hydrogen production process, named a hybrid thermo-chemical process, which is based on H2SO4 synthesis and decomposition, and SO3 electrolysis with a solid electrolyte at 500-550 degrees C. In order to improve the energy efficiency, it is necessary to reduce SO3 electrolysis resistance. For refining the rate determining process of SO3 electrolysis, we performed SO3 electrolysis with YSZ solid electrolyte, and it was found that YSZ solid electrolyte resistance was small enough to be regarded as unimportant, and that SO3 electrolysis resistance was almost due to the electrode reaction, which meant the rate determining process was involved in the electrode reaction. In order to analyze this experiment, we created an analytical model. With this model, we concluded that the rate determining process was the adsorption and dissociation reaction near the three phase boundary at 450 500 degrees C and the surface diffusion on the electrode at 500-600 degrees C, and it was important to encourage the adsorption and dissociation reaction near the three phase boundary in order to improve SO3 electrolysis in low temperature (450-500 degrees C) or SO3 high pressure (0.5-1.0 atm) range. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.