Highly dispersed Mo6+-oxide catalysts were supported onto various oxides (SiO2, Al2O3, and SiO2-Al2O3) by an impregnation method and their photocatalytic reactivities were investigated for the decomposition of NO with CO as well as the preferential oxidation of CO with O-2 in the presence of excess H-2 (photo-PROX). MO/SiO2 was found to show the highest photocatalytic activity for these reactions. UV-vis and XAFS investigations revealed that isolated tetrahedral Mo6+-oxide species are the main Mo-oxide moieties on these catalysts. Furthermore, the distortion of the coordination sphere of tetrahedral Mo6+-oxide species was observed to increase in the following order: Mo/Al2O3 < MO/SiO2-Al2O3 < Mo/SiO2, showing that the distorted tetrahedral Mo6+-oxide species having two short Mo=O double bonds is the active species for these reactions. Photoluminescence and FT-IR investigations indicated that the photo-exited Mo6+-oxide species (Mo5+O-)* efficiently reacted with CO to form CO2 and the Mo4+-carbonyl species which is easily re-oxidized into the original Mo6+-oxide species (Mo6+=O2-) through reactions with NO, N2O and O-2 under dark conditions. Such unique redox properties of the Mo6+-oxide species were found to play a significant role in the photocatalytic reactions. (c) 2007 Elsevier B.V. All rights reserved.