The electronic and structural properties of sexiphenyl and sexithiophene films grown under ultra-high vacuum conditions on a variety of well-controlled substrate surfaces have been examined. We show that, in contradiction with the general notion, the ionisation potential of the organic films is not a material constant. Considering the variable ionisation potential, the validity of the Schottky-Mott relationship, which expresses a dependence of the band alignment on the substrate work function, is analysed. We also briefly discuss the relevance of the built-in potential behaviour in organic films for the band-bending concept. (c) 2006 Elsevier B.V. All rights reserved.