Two-step thermochemical cycle for CO2 splitting has been studied using cerium oxide (CeO2-delta) as a redox material. Since oxygen release capacity of CeO2-delta increases by doping ZrO2, a lot of studies have been conducted on CeO2-ZrO2 solid solutions as state of the art redox materials for this cycle. In this study, further doping of Sc3+ into CeO2-ZrO2 solid solution was attempted with an expectation to increase amount of available oxygen vacancies which could improve redox kinetics. Scandium was chosen since the ionic radius is close to Zr4+ and oxygen vacancy is introduced by the aliovalent doping. Oxygen nonstoichiometry of Ce-0.(8) Zr0.15Sc0.05O2-delta was investigated by a coulometric titration method and chemical diffusion coefficient and surface reaction rate constant by an electrical conductivity relaxation method. Oxygen vacancy formation of Ce-0.(8) Zr0.15Sc0.05O2-delta upon reduction was similar to that of Ce-0.(8) Zr0.2O2-delta, while chemical diffusion coefficient in a high oxygen partial pressure region became larger by introducing Sc3+. Those were discussed in correlation with the reaction kinetics in the thermochemical cycle.