SiO2-impregnated complete and incomplete cubane-type molybdenum oxide clusters such as [(RhCp*)(4)Mo4O16] (I) and [(RhCp*)(2)Mo3O9(OMe)(4)] (II) were photoreduced with a UHP-Hg lamp (> 365 nm) in CO, resulting in the formation of catalytically active species for propene metathesis at 300 K. The CO-photoreduced Mo oxide clusters were characterized by EXAFS, IR, XPS and TPD technique. The results suggest that the Mo-O-4c (four-centered bridging oxygens) in I were specifically reduced with CO under the illumination to produce two sets of subcarbonyl species, e.g., those characteristic of the IR bands at 2061 and 2021 cm(-1), which were thermally inactive for the (CO)-C-13 exchange reaction but very active under illumination at 300 K, possibly assignable to Mo(CO)(x) (x = 2, 3), whereas those at 2092 and 2035 cm(-1) due to Rh(CO)(2), which were readily exchangeable with (CO)-C-13 at 300 K in dark. Removal of both carbonyls attached on Rh and Mo in photoreduced I and II by evacuation at 375-440 K led to the formation of oxygen-deficient Mo4+/Mo5+ sites, which exhibited high catalytic activities in propene metathesis at 300 K to produce an equimolar mixture of ethene and 2-butenes. The CO-photoreduced incomplete cubane Mo oxide cluster (II) exhibited higher activities and higher trans/cis ratios of 2-butenes in the reaction, compared with those on the photoreduced I.