A silica gel for chromatography was found to increase its photocatalytic activity for the CO oxidation reaction with O/sub 2/ under UV irradiation during the progress of the reaction, whereas no such enhancement was obtained on irradiation under vacuum or in the presence of O/sub 2/, CO, or CO/sub 2/ alone. The surface of the photoactivated silica gel was characterized in comparison with the nonactivated one by the formation of several paramagnetic species, followed by using an ESR spectroscopic technique. On the photoactivated samples, an anomalous type of O/sub 3//sup -/ ion, different from the ozonide, CO/sub 2//sup -/, C/sub 2/H/sub 4/O/sup -/, and SO/sub 3//sup -/ ions, was formed in O/sub 2/, CO, C/sub 2/H/sub 4/, and SO/sub 2/ atmospheres, respectively, under UV irradiation. This might suggest that a hole center of O/sup -/ was readily developed from the surface lattice oxygen under UV irradiation. The nonactivated silica gel, on the other hand, gave mainly O/sub 2//sup -/ or SO/sub 2//sup -/ ions in O/sub 2/ or SO/sub 2/, indicating that surface electron transfer had occurred. It was also found that the formed O/sub 3//sup -/ ions reacted thermally with CO to give CO/sub 4//sup -/ at 77 K,more » followed by a thermal decomposition of CO/sub 4//sup -/ to CO/sub 2/ and O/sub 2//sup -/ upon warming the sample to room temperature. The reaction mechanism of the CO photooxidation with O/sub 2/ on the photoactivated silica gel is discussed on the basis of these findings.« less