Carbon dioxide (CO2) is one of the stable and inactive molecules that contribute to greenhouse gases. The development of new reactions of CO, activation, chemical fixation, and conversion is a very important issue. In this report, the reactions of CO2-H2O binary clusters were investigated using a direct ab initio molecular dynamics (AIMD) method to find a new reaction of CO, activation. Clusters composed of carbon dioxide and water molecules, CO2 (H2O)(n) (n = 2-5), were utilized as a model of the binary cluster. The reaction dynamics of [CO2 (H2O)(n)](+) following the ionization of parent neutral clusters were also investigated. Two electronic states of [CO2(H2O)(n)](+) were examined for direct AIMD surfaces: [CO2(H2O)(n)](+) (ground state) and (CO2 ) + (H2O)(n) (excited charge transfer (CT) state). After the ionization of the clusters, a proton-transfer (PT) reaction occurred within the (H2O)(n)(+) moiety at the ground state, whereas the reactive HCO3 radical was formed at the CT state for OH addition to CO2+ : + CO2+(H2O)(n) -> HCO3 +H- (+)(H2O)(n-)(1). The mechanisms of the PT process and the HCO3 radical formation were discussed based on the theoretical results.