A high surface area Co3O4-SiO2 nanocomposite catalyst has been prepared by use of activated carbon as template. The Co3O4-SiO2 composite, the surface of which is rich in silica and Co (II) species compared with normal Co3O4, exhibited very high activity for CO oxidation even at a temperature as low as -76 degrees C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 degrees C, was observed with a normal feed gas (H2O content similar to 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 Co3O4-SiO2 catalyst led to the conclusion that adsorption of H2O 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.