The kinetics of the electron self-exchange reaction of the redox couple Re2X4(PMe2Ph)(4)/[Re2X4(PMe2Ph)PF6 (X = Cl, Br) has been measured as a function of temperature and reactant concentration in methylene chloride by the H-1 NMR line-broadening method. The self-exchange rate constants were obtained from broadening of the methyl singlet on the phosphine ligand in the fast-exchange limit. Extrapolated to 298 K from lower temperatures, they are 2.3 x 10(8) M-1 s(-1) for X = Cl and 4.2 x 10(8) M-1 s(-1) for X = Br. The corresponding activation parameters are Delta H-double dagger = 9.3 +/- 0.6 kcal/mol and Delta S-double dagger = 10.9 +/- 2.2 cal/(mol K) for X = Cl and Delta H-double dagger = 8.1 +/- 0.7 kcal/mol and Delta S-double dagger = 8.1 +/- 2.9 cal/(mol K) for X = Br. As expected, on the basis of previous cross-reaction studies, the reactivity of the dirhenium complexes is high, The previously calculated apparent self-exchange rate constant from cross-reaction studies for Re2Br4(PMe2Ph)(4) (0/+) was 2.5 x 10(8) M-1 s(-1) (298 K). The inner- and outer-sphere reorganization energies for Re2Cl4(PMe2Ph)(4) are estimated to be 1.3 and 4.4 kcal/mol, respectively, indicating a large contribution to the rate constant from solvation. These results are consistent with studies of the heterogeneous electron transfer of these complexes and are compared to studies of other metal complexes in methylene chloride.