The rates of intramolecular electron transfer in T65C cytochrome b(5) labeled with three different ruthenium polypyridine complexes at Cys-65 have been measured over a range of ionic strengths in the presence and absence of cytochrome c. The redox potentials of the ruthenium complexes provide for a range of free energies of reaction which span the reorganizational energy of the system. The system obeys the familiar dependence of In k(obs) on free energy of reaction predicted by Marcus (Marcus, R. A. J. Chem. Phys. 1956, 24, 966). The reorganization energies for these reactions obtained at 0 and 300 mM NaCl in the absence of cytochrome c are 1.02 and 0.93 eV, respectively. The corresponding electronic coupling elements calculated as H-AB from k(max) = 2.0 x 10(7) and 1.4 X 10(7) s(-1) are 0.29 and 0.21 cm(-1), respectively. Surprisingly, the rate constants obtained with cytochrome c bound to cytochrome b(5) ([NaCl] = 0) are the same as those obtained at [NaCl] > 150 mM in the absence of cytochrome c. These rate constants are, however, very different from those obtained at low [NaCl] in the absence of cytochrome c. We suggest that these results can be explained in terms of an alteration of the solvent reorganizational barrier either by binding of cytochrome c and exclusion of water from the binding domain or by the influence of sodium and chloride ions on the solvent in close proximity to the redox centers.