Intentional variations of the driving force for back electron transfer from titanium dioxide to a surface-bound redox couple, via synthetic alteration of the couple's formal potential, show that the reaction takes place in the Marcus normal region; i.e., rates become faster as the driving force increases. Variable-temperature rate measurements show that back ET is thermally activated, with the activation barrier decreasing with increasing driving force, as expected for a normal region process. The observation of normal region behavior, despite the existence of overall reaction driving forces that significantly exceed the likely reorganization energy, is attributed to the occurrence of a sequential electron-and proton-transfer process. The sequential process yields a driving force for the rate-determining electron-transfer step that is considerably smaller than the overall reaction driving force. The sequential electron-and proton-transfer mechanism also provides an explanation for the pH independence of the back-ET kinetics. Variable-temperature rate measurements additionally point toward a high degree of nonadiabaticitys3-5 orders of magnitude of rate attenuation due solely to inefficient electronic coupling. The physical basis for the effect presumably is in the need to traverse eight bonds, some of them saturated, in the back-ET process. Together with the barrier activation effects, the reaction nonadiabaticity accounts for the slow rates for back ET (ca. 10 7 s -1) and, therefore, much of the ability of metal-to-ligand charge-transfer type chromophores to function effectively as sensitizers in TiO 2 -based photoelectrochemical cells. The findings also suggest that more efficient cells could be constructed by extending the chemical linkage between the dye and semiconductor and by further decreasing the driving force for the back-ET process.