Electron-transfer reactions from nanoparticle TiO2 films to outer-sphere redox shuttles were investigated. Steady-state dark current density versus applied potential and open circuit voltage decay measurements were employed to determine the rates of recombination to cobalt(III) tris(4,4'-dimethyl-2,2'-bipyridyl), [Co(Me(2)bpy)(3)](3+), and ruthenium(III) bis(2,2'-bipyridyl)-bis (N-methylimidozole), [Ru(bpy)(2)(MeIm)(2)](3+). A striking difference in the magnitude as well as the shape of the electron lifetimes for TiO2 electrodes in contact with these two redox shuttles is observed. A model based on Marcus theory is developed to describe recombination, including contributions from conduction band electrons and surface states. Excellent agreement was found between the modeled and measured lifetimes. The model allows for identification of each contributing component of electron transfer to the measured lifetimes. Comparison of the different components of the modeled lifetimes to the measured lifetimes provides dear evidence for recombination mediated through surface states.