Although the importance of a Au/oxide perimeter interface has been recognized in the field of gold catalysis, the experimental evidence on the reaction occurring at the perimeter is still limited. In this research, we applied in situ electrical conductance measurement (in situ ECM) to measure the CO oxidation with O-2 over Au/TiO2 catalyst powder and found that the oxygen vacancies are generated at the Au/TiO2 perimeter interface during the CO oxidation. The present result supports the reaction model in which oxygen molecules are activated on such oxygen vacancies.