Photoinduced, interfacial electron injection and back electron transfer between surface-bound [Ru-II(bpy)(2)(4,4'-(PO3H2)(2)-bpy)](2+) and degenerately doped In2O3:Sn nanoparticles, present in mesoporous thin films (nanoITO), have been studied as a function of applied external bias. Due to the metallic behavior of the nanoITO films, application of an external bias was used to vary the Fermi level in the oxide and, with it, the driving force for electron transfer (Delta G degrees'). By controlling the external bias, Delta G degrees' was varied from 0 to -1.8 eV for electron injection and from -0.3 to -1.3 eV for back electron transfer. Analysis of the back electron-transfer data, obtained from transient absorption measurements, using MarcusGerischer theory gave an experimental estimate of lambda = 0.56 eV for the reorganization energy of the surface-bound Ru-III/(II) couple in acetonitrile with 0.1 M LiClO4 electrolyte.