Pure-phase alpha-MnO2 and beta-MnO2 nanowires/nanorods were synthesized through an easy solution-based hydrothermal method, and the effect of the manganese dioxide phase on the activity of MnO2 and Ag/MnO2 for the oxidation of CO was investigated. MnO2 is an effective catalyst in CO oxidation, and its activity depends on the crystal phase of MnO2. alpha-MnO2 exhibits a higher activity than beta-MnO2, because the alpha-MnO2 nanowires can be reduced more easily than the beta-MnO2 nanorods. Moreover, when Ag was introduced to MnO2, a strong interaction Occurred between Ag and MnO2. The catalytic activity clearly correlates with this interaction, which is determined by crystal phase and surface structure. (c) 2005 Elsevier Inc. All rights reserved.