Ultraviolet photoelectron spectroscopy has been used to investigate the interface between thermally deposited alpha-sexithiophene (6T) and buckminsterfullerene (C60) films on gold surfaces. Charge transfer from 6T to C60 is evidenced by shifts in the interfacial C60 (6T) highest occupied molecular orbitals toward lower (higher) ionization energies. The absence of vacuum level alignment with an offset of 0.6 eV +/- 0.05 eV is observed, independent of the order of deposition. The vacuum level offset matches the difference in work functions between the C60/Au and 6T/Au surfaces, indicating that the vacuum levels of the organic layers remain pinned to the Fermi level. (c) 2007 Elsevier B.V. All rights reserved.