Photochemical CO2 reduction sensitized by rhenium bipyridyl complexes has been studied through multiple approaches during the past several decades. However, a key reaction intermediate, the CO2-coordinated Re-bipyridyl complex, which should govern the activity of CO2 reduction in the photocatalytic cycle, has never been detected in a direct way. In this study on photoreduction of CO2 catalyzed by the 4,4'-dimethyl-2,2'-bipyridine (dmbpy) complex, [Re(dmbpy)(CO)(3)Cl] (1), we successfully detect the solvent-coordinated Re complex [Re(dmbpy)(CO)(3)DMF] (2) as the light-absorbing species to drive photoreduction of CO2. The key intermediate, the CO2-coordinated Re bipyridyl complex, [Re(dmbpy)(CO)(3)(COOH)], is also successfully detected for the first time by means of cold-spray ionization spectrometry (CSI-MS). Mass spectra for a reaction mixture with isotopically labeled (CO2)-C-13 provide clear evidence for the incorporation of CO2 into the Re bipyridyl complex. It is revealed that the starting chloride complex 1 was rapidly transformed into the DMF-coordinated Re complex 2 through the initial cycle of photoreduction of CO2. The observed induction period in the time profile of the CSI-MS signals can well explain the subsequent formation of the CO2-coordinated intermediate from the solvent-coordinated Re bipyridyl complex. An FTIR study of the reaction mixture in dimethyl sulfoxide clearly shows the appearance of a signal at 1682 cm(-1), which shifts to 1647 cm(-1) for the (CO2)-C-13-labeled counterpart; this is assigned as the CO2-coordinated intermediate, Re-II-COOH. Thus, a detailed understanding has now been obtained for the mechanism of the archetypical photochemical CO2 reduction sensitized by a Re-bipyridyl complex.