Using ultraviolet photoemission spectroscopy (UPS), we show that the conformation of a conjugated organic molecule can be changed significantly upon adsorption on a metal surface, even in the case of weak interactions with the metal. This is accomplished by comparing the thickness-dependent spectral evolution for a non-rigid non-planar molecule (p-sexiphenyl) deposited onto Ag(111), with those of a coplanar molecule (alpha-sexithienyl) and a rigid non-planar one (1,3,4,6,7,9,10,12,13,15,16,18-dodecamethyl-hexa-peri-hexabenzocoronene) . Molecular conformation changes can influence hole injection barriers by at least 0.1 eV. These results have a direct impact on the detailed understanding of organic/metal interfaces. (c) 2005 Elsevier B.V. All rights reserved.