A theoretical scheme is presented to study the rate of the electron-exchange reaction between transition metal complexes in aqueous solution. In the scheme the activation energy is obtained via an activation model and ab initio technique, and the coupling matrix element at the transition state is determined by using perturbation theory and a numerical integral method. The electron-transfer (ET) reactivity of Co(H2O)62+/3+, a prototype system of the ET reactions between transition metal complexes in solution, is determined by employing this scheme. The theoretical results are compared with the corresponding experimental values and good agreement is found. The further extension of the scheme is also discussed.