We study quenching mechanism of long-lived photoexcitations and interfacial electronic structures of photo-oxidized MEH-PPV on gold using two-photon photoemission spectroscopy, in comparison with unannealed and annealed films. The work function of 3.95 eV is not changed by photo-oxidation while the yield and kinetics of photoexcitations are significantly influenced by photo-oxidation. Upon photo-oxidation, we observed narrowing of photoemission distribution at higher photon energy, attributed to lowering of an intermediate state level and/or quenching of the photoexcitation state. The photoemission yield at higher-lying photoexcitations quenches dramatically, understood by charge transfer toward defects, consistent with first-order kinetics - increase in power law slope. (c) 2007 Elsevier B.V. All rights reserved.