A mechanistic investigation on the photocatalytic reduction of CO2 with hexagonal CdS nanocrystallites prepared in N,N-dimethylformamide (DMF) was carried out from the standpoint of surface structures of the nanocrystallites. A remarkable increase of photocatalytic activity could be achieved by addition of excess Cd2+ to the system. Analysis of the emission behavior depending on the amount of excess Cd2+ in the system suggests that the Cd2+ addition results in the formation of sulfur vacancies on the surface of nanocrystallites due to the adsorption of excess Cd2+ to the surface. The formation of the sulfur vacancies on the surface was supported by in situ Cd K-edge EXAFS analysis of the nanocrystallites in solution as changes in the coordination numbers of cadmium-sulfur and cadmium-oxygen. Theoretical MO calculations using a density functional (DF) method supported the preferential bidentate-type absorption of CO2 with the Cd atom in the vicinity of the sulfur vacancy.