Kinetic studies of the electron exchange reaction between Cu(bcp) 2 2 + and Cu(bcp) 2 + were carried out in acetonitrile (bcp = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline). The electron self-exchange rate constant was k e x (25°C) = 1.4 10 3 kg mol - 1 s - 1 with ΔH * = 38.1 ± 2.4 kJ mol - 1 and ΔS * = -57 ± 8 J mol - 1 K - 1 (ionic strength 0.1 mol kg - 1 (tetraethylammonium tetrafluoroborate)). A small dependence of the rate constant on the ionic strength was observed. Electron spin resonance study of the Cu(II) species revealed that the Cu-N distances and the dihedral angles between the two N-Cu-N planes in Cu(bcp) 2 2 + and Cu(dmp) 2 2 + complexes (dmp = 2,9-dimethyl-1,10-phenanthroline) are identical in solution. Although the comparison of the crystal structures suggests that the dihedral angle in the Cu(dmp) 2 + complex is closer to 90° than that of the Cu(bcp) 2 + complex, there was no significant difference between the structures of these complex ions in solution. The electron self-exchange rate constants for the Cu(bcp) 2 2 + / + and the Cu(dmp) 2 2 + / + couples were similar to each other, despite the difference in radii of these ions.