[Sn(acac)(2)]Cl-2 is chemisorbed on the surfaces of anatase TiO2 via 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 (similar to 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 FeOx 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 O-2 to increase the photocatalytic activity. ; Science Foundation Ireland (Starting Investigator Research Grant Program, project ‘‘EMOIN’’, grant number SFI 09/SIRG/I1620); Science Foundation Ireland (Irish Centre for High End Computing); Higher Education Authority (Irish Centre for High End Computing); Ministry of Education, Science, Sport, and Culture, Japan (Grant-in-Aid for Scientific Research (B) No. 20350097)