The vertical bonding distance of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) above the Au(1 1 1) surface has been mea-sured by the normal incidence X-ray standing wave (NIXSW) technique. The carbon skeleton of PTCDA has a vertical distance of D = (3.27 ± 0.02) Å to the Au(1 1 1) substrate. This distance corresponds very nearly to the sum of the van der Waals radii of carbon and gold, suggesting the adsorption to be a physisorptive one. In contrast, the PTCDA/Ag(1 1 1) interface which according to spectro-scopic data follows the standard model of chemisorption very closely, shows a considerably smaller bonding distance of D = (2.86 ± 0. 209206]. The different vertical adsorption heights of PTCDA on gold and silver are discussed in relation to the different bonding mechanisms on both noble metal surfaces.