A new crossed beam instrument for the investigation of ion-molecule reactions is described. The reaction CO++O2→CO2++O has been studied over the range of initial relative translational energies from 1·8 to 10 eV. The cross-section increases as the energy is raised to 5 eV and then decreases as the energy is increased further. The reaction proceeds by a direct mechanism at all energies, though at low energies there are substantial interactions between all four atoms. As the initial energy is raised the mechanism becomes more impulsive and there is evidence for the formation of products in an excited electronic state.