A high surface area Co(3)O(4)-SiO(2) nanocomposite catalyst has been prepared by use of activated carbon as template. The Co(3)O(4)-SiO(2) composite, the surface of which is rich in silica and Co(II) species compared with normal Co(3)O(4), exhibited very high activity for CO oxidation even at a temperature as low as -76 °C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 °C, was observed with a normal feed gas (H(2)O content ~3 ppm). The U-shape of the activity curve indicates a negative apparent activation energy over a certain temperature range, which has rarely been observed for the heterogeneously catalyzed oxidation of CO. Careful investigation of the catalytic behavior of Co(3)O(4)-SiO(2) catalyst led to the conclusion that adsorption of H(2)O molecules on the surface of the catalyst caused the unusual behavior. This conclusion was supported by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopic experiments under both normal and dry conditions.