The valence spectra of monolayers of ferrocene oligo(phenylene vinylene) (OPV) methyl thiols deposited on gold have been collected using synchrotron radiation. These spectra reveal the relative positions of the gold Fermi level, the highest occupied molecular orbital of ferrocene, and an upper limit for the highest occupied molecular orbital of OPV. The onset of ejected electrons from ferrocene occurs 0.2 eV below the Fermi level of gold, and the onset of ejected electrons from OPV occurs at least 0.8 eV below the Fermi level of gold. The difference between these values implies a hole injection barrier from ferrocene to OPV of at least 0.6 eV. Such a large injection barrier rules out a hopping mechanism for electron transfer between gold and ferrocene through OPV.