The thermochem. of the RuO42- + MnO4- RuO4- + MnO42- redox reaction in aq. soln. is studied by sep. d.-functional-based ab initio mol.-dynamics simulations of the component half reactions RuO42- -> RuO4- + e- and MnO42- -> MnO4- + e-. We compare the results of a recently developed grand-canonical method for the computation of oxidn. free energies to the predictions by the energy-gap relations of the Marcus theory that can be assumed to apply to these reactions. The calcd. redox potentials are in good agreement. The subtraction of the half-reaction free energies gives an est. of the free energy of the full reaction. The result obtained from the grand-canonical method is -0.4 eV, while the application of the Marcus theory gives -0.3 eV. These should be compared to the exptl. value of 0.0 eV. Size effects, in response to increasing the no. of water mols. in the periodic model system from 30 to 48, are found to be small ( ~ 0.1 eV). The link to the Marcus theory also has enabled us to compute reorganization free energies for oxidn. For both the MnO22- and RuO42- redox reactions we find the same reorganization free energy of 0.8 eV (1.0 eV in the larger system). The results for the free energies and further anal. of solvation and electronic structure confirm that these two tetrahedral oxoanions show very similar behavior in soln. in spite of the central transition-metal atoms occupying a different row and column in the periodic table. [on SciFinder (R)]