The chromium carbonyl species that form during ultraviolet (UV) photoreduction of Cr6+/SiO2 in (CO)-C-12-O-16 and (CO)-C-13-O-16 were analyzed by Fourier transform infrared spectroscopy (FTIR). Four carbonyl species were identified : linear Cr4+(CO), mer-Cr2+(CO)(3), linear Cr2+(CO)(2) and Cr-2(CO)(2) (band at 2099 cm(-1)). The linear Cr4+(CO) band at 2206 cm(-1), formed at short UV exposures, increased to a maximum and decreased at long UV exposures. Both the mer-Cr2+(CO)(3), with bands at 2212, 2190, and 2178 cm(-1), and linear Cr2+(CO), with a band at 2184 cm(-1), were present at all UV exposures. The IR bands arising from these two species were isolated during isothermal evacuation. The oxidation state of the photoformed Cr2+ species was assigned by comparing the IR bands following thermal reduction of Cr6+/SiO2 in CO to those formed following photoreduction.