[Sn(acac)₂]Cl₂ is chemisorbed on the surfaces of anatase TiO₂via ion-exchange between the complex ions and H⁺ released from the surface Ti–OH groups without liberation of the acetylacetonate ligand (Sn(acac)₂/TiO₂). The post-heating at 873 K in air forms tin oxide species on the TiO₂ surface in a highly dispersed state on a molecular scale ((SnO₂)_m/TiO₂). A low level of this p block metal oxide surface modification (∼0.007 Sn ions nm⁻²) accelerates the UV-light-activities for the liquid- and gas-phase reactions, whereas in contrast to the surface modification with d block metal oxides such as FeO_x and NiO, no visible-light response is induced. Electrochemical measurements and first principles density functional theory (DFT) calculations for (SnO₂)_m/TiO₂ model clusters (m = 1, 2) indicate that the bulk (TiO₂)-to-surface interfacial electron transfer (BS-IET) enhances charge separation and the following electron transfer to O₂ to increase the photocatalytic activity.