Interfacial electron transfer to and from conductive Sn-doped In2O3 (ITO) nanoparticles (NPs) in mesoporous thin films has been investigated by transient absorption measurements using surface-bound [Ru-II(bpy)(2)(dcb)](2+) (bpy is 2,2'-bipyridyl and dcb is 4,4'-(COOH)(2)-2,2'-bipyridyl). Metal-to-ligand charge transfer excitation in 0.1 M LiClO4 MeCN results in efficient electron injection into the ITO NPs on the picosecond time scale followed by back electron transfer on the nanosecond time scale. Rates of back electron transfer are dependent on thermal annealing conditions with the rate constant increasing from 1.8 x 10(8) s(-1) for oxidizing annealing conditions to 8.0 x 10(8) s(-1) for reducing conditions, presumably due to an enhanced electron concentration in the latter.