Photocatalytic reduction of CO on TiO and Cu/TiO photocatalysts was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under UV irradiation. The photocatalysts were prepared by sol-gel method via controlled hydrolysis of titanium (IV) butoxide. Copper precursor was loaded onto TiO during sol-gel procedure. A large amount of adsorbed HO and surface OH groups was detected at 25°C on the TiO photocatalyst after being treated at 500°C under air stream. Carbonate and bicarbonate were formed rapidly due to the reaction of CO with oxygen-vacancy and OH groups, respectively, on TiO surface upon CO adsorption. The IR spectra indicated that, under UV irradiation, gas-phase CO further combined with oxygen-vacancy and OH groups to produce more carbonate or bicarbonate. The weak signals of reaction intermediates were found on the IR spectra, which were due to the slow photocatalytic CO reduction on photocatalysts. Photogenerated electrons merge with H ions to form H atoms, which progressively reduce CO to form formic acid, dioxymethylene, formaldehyde and methoxy as observed in the IR spectra. The well-dispersed Cu, acting as the active site significantly increases the amount of formaldehyde and dioxymethylene, thus promotes the photoactivity of CO reduction on Cu/TiO. A possible mechanism of the photocatalytic CO reduction is proposed based on these intermediates and products on the photocatalysts.