CO and formaldehyde (HCHO) oxidation reactions were investigated over mesoporous Ag/Co 3 O 4 catalysts prepared by one-pot (OP) and impregnation (IM) methods. It was found that the one-pot method was superior to the impregnation method for synthesizing Ag/Co 3 O 4 catalysts with high activity for both reactions. It was also found that the catalytic behavior of mesoporous Co 3 O 4 and Ag/Co 3 O 4 catalysts for the both reactions was different. And the addition of silver on mesoporous Co 3 O 4 did not always enhance the catalytic activity of final catalyst for CO oxidation at room temperature (20 °C), but could significantly improve the catalytic activity of final catalyst for HCHO oxidation at low temperature (90 °C). The high surface area, uniform pore structure and the pretty good dispersion degree of the silver particle should be responsible for the excellent low-temperature CO oxidation activity. However, for HCHO oxidation, the addition of silver played an important role in the activity enhancement. And the silver particle size and the reducibility of Co 3 O 4 should be indispensable for the high activity of HCHO oxidation at low temperature.