A series of cationic Ir(III) complexes ([Ir(btp)(2)(bpy-X-2)](+) (Ir-X+ btp = (2-pyridyl)benzo[b]thiophen-3-yl; bpy-X-2 = 4,4'-X(2)2,2'- (X = OMe, Bu-t, Me, H, and CN)) were applied as visible-light photosensitizer to the CO2 reduction to CO using a hybrid catalyst (TiO2/ReP) prepared by anchoring of Re(4,4'-Y-2-bpy)(CO)(3)Cl (Rd); Y = CH2PO(OH)(2)) on TiO2 particles. Irradiation of a solution containing Ir-X+, TiO2/ReP particles, and an electron donor (1,3-dimethy1-2-phenyl-1,3-dihydrobenzimidazole) in N,N-dimethylformamide at greater than 400 nm resulted in the reduction of CO2 to CO with efficiencies in the order X = OMe > Bu-t approximate to Me > H; Ir-CN+ has no photosensitization effect. A notable observation is that Ir-Bu-t(+) and Ir-Me+ are less efficient than Ir-OMe+ at an early stage of the reaction but reveal persistent photosensitization behavior for a much longer period of time unlike the latter. Comparable experiments showed that (1) the Ir-X+ sensitizers are commonly superior compared to Ru(bpy)(3)(2+), a widely used transition -metal photosensitizer, and (2) the system comprising Ir-OMe+ and TiO2/ReP is much more efficient than a homogeneous -solution system using Ir-OMe+ and Re(4,41-Y'(2)-bpy)(CO)(3)Cl (Y' = CH2PO(OEt)(2)). Implications of the present observations involving reaction mechanisms associated with the different behavior of the photosensitizers are discussed in detail.