Copper and iodine co-modified TiO2 nanoparticles (Cu-I-TiO2) were synthesized through a combined hydrothermal and wet-impregnation process. The structures and properties of the catalysts were characterized by XRD, BET, SEM/EDX, XPS, and UV-vis diffuse reflectance spectroscopy. Iodine ions were doped in the TiO2 lattice by replacing Ti4+ and, consequently, Ti3+ was generated to balance the charge. Iodine doping reduced the TiO2 crystal size and was responsible for visible light absorption. Cu species were found to deposit on the surface of TiO2 and resulted in a slightly increased particle size. The activity of the Cu-I-TiO2 catalyst was investigated by the photocatalytic reduction of CO2 with water vapor, and CO was found to be the major reduction product with trace amounts of CH4 generated. Under UV-vis irradiation, the activity of the co-modified catalyst (Cu-I-TiO2) was higher than that of the single ion-modified catalysts (Cu-TiO2 or I-TiO2). Under visible light irradiation, the addition of Cu to I-TiO2 did not lead to significant improvements in CO2 reduction. Methyl chloride (CH3Cl) was detected as a reaction product when CuCl2 was used as the precursor in the synthesis, thus suggesting that methyl radicals are reaction intermediates. When CuCl2 was used as the Cu precursor, a three-fold increase in CO2 photoreduction activity was observed, as compared to when Cu(NO3)(2) was used as the Cu precursor. These differences in activities were probably due to enhanced Cu dispersion and the hole-scavenging effects of the Cl ions. However, the formation of by-products (e.g., CH3Cl) may be undesirable. (C) 2012 Elsevier B.V. All rights reserved.