The kinetics of the electron transfer reactions between [CO(C2O4)3](3-) and different ruthenium complexes [Ru(NH3)(5)L](2+) (L = pyridine, pyrazine and 4-cyanopyridine) have been studied in water-methanol mixtures. A method for the evaluation of the reorganization energy of these reactions, based on comparison of their rate constants, is proposed. The results showed a decrease of the rate constants and a decrease of the reorganization energies when the dielectric constant of the medium decreases. This behaviour is according to that obtained from the use of classical continuum methods. The slight differences found between the distinct methods could be related to the existence of specific effects of the medium. (c) 2005 Elsevier B.V. All rights reserved.