The origin of the exceptionally high activity of (B, Ag)-codoped TiO2 catalysts under solar-light irradiation has been investigated by XPS and B-11 solid-state NMR spectroscopy in conjunction with density functional theory (DFT) calculations. XPS experimental results demonstrated that a portion of the dopant Ag (Ag3+) ions were implanted into the crystalline lattice of (B, Ag)-codoped TiO2 and were in close proximity to the interstitial B (B-int) sites, forming [B-int-O-Ag] structural units. In situ XPS experiments were employed to follow the evolution of the chemical states of the B and Ag dopants during UV-vis irradiation. It was found that the [B-int-O-Ag] units could trap the photoinduced electron to form a unique intermediate structure in the (B, Ag)-codoped TiO2 during the irradiation, which is responsible for the photoinduced shifts of the B Is and Ag 3d peaks observed in the in situ XPS spectra. Solid-state NMR experiments including B-11 triple-quantum and double-quantum magic angle spinning (MAS) NMR revealed that up to six different boron species were present in the catalysts and only the tricoordinated interstitial boron (T*) species was in close proximity to the substitutional Ag species, leading to formation of [T*-O-Ag] structural units. Furthermore, as demonstrated by DFT calculations, the [T*-O-Ag] structural units were responsible for trapping the photoinduced electrons, which prolongs the life of the photoinduced charge carriers and eventually leads to a remarkable enhancement in the photocatalytic activity. All these unprecedented findings are expected to be crucial for understanding the roles of B and Ag dopants and their synergistic effect in numerous titania-mediated photocatalytic reactions.