In this kinetic and thermodynamic study, the reversible outer-sphere electron-transfer reactions between a series of Ru(NH3)(5)L(3+/2+) complexes (L = etpy, py, lut) (etpy = 4-ethylpyridine; py = pyridine; lut = 3,5-lutidine) and cytochrome c were investigated as a function of ionic strength, buffer, pH, temperature, and pressure. Due to the low driving forces of these systems, it was possible to study all the reactions in both redox directions. The observed rate constants for various L are correlated on the basis of the ability of ligands on the ruthenium complex to penetrate the heme groove on cytochrome c. The measurements as a function of pressure enabled the construction of volume profiles for all investigated systems. The activation volumes for all of these processes are very similar : between -14.9 and -17.8 cm(3) mol(-1) for the reduction and between +14.7 and +17.8 cm(3) mol(-1) for the oxidation of the protein by Ru(MH(3))(5)L(2+/3+), respectively. The overall reaction volume varies between 27 and 35 cm(3) mol(-1), from which it follows that the transition state lies exactly halfway between reactant and product states on a volume basis in all cases. There is good agreement throughout between kinetic and thermodynamic data.