The tungsten carbonyl species that form during ultraviolet photoreduction of W6+/SiO2 in CO were analyzed by Fourier transform infrared spectroscopy (FTIR). Two carbonyl species were identified, mer-W(CO)(3) and cis-W(CO)(2), by the number and intensities of the respective IR peaks and through isotopic substitution. The C-2v symmetry of mer-W(CO)(3) was associated with three IR bands : a very weak (A(1))(1) symmetric trans mode at 2145 cm(-1), a strong B-2 symmetric mode at 2112 cm(-1), and a strong (A(1))(2) antisymmetric mode at 2179 cm(-1) mer-W(CO)(3) was stable at 298 K in 25 Torr of CO. Isothermal evacuation of the mer-W(CO)(3) species resulted in the loss of a single CO ligand, forming cis-W(CO)(2). The W(CO)(2) species formed during both isothermal evacuation of the mer-W(CO)(3) species and during the initial photoreduction process. cis-W(CO)(2) was identified by its IR spectrum consisting of a strong symmetric mode at 2112 cm(-1) and a strong antisymmetric mode at 2040 cm(-1).