We present a Raman spectroscopic analysis of the bonding properties of organic molecules on Ag(1 1 1) surfaces as a model system for organic semiconductor/metal contacts. The planar molecule 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA) is used in the present study due to its high electronic mobility and thermal stability. Vibrational signatures of the first monolayer in direct contact with the metal, especially the modes at 1310 and 1575 cm(-1), are found to be shifted to lower frequencies. First principles calculations result in corresponding mode patterns, which are essentially located in the centre of the molecule thus indicating the area, where major metal interaction takes place. Furthermore, upon annealing at 450 K the vibrational frequencies of the second monolayer are modified with respect to bulk PTCDA, which reflect particular changes of the bonding situation of this layer. For subsequent layers bulk-like behaviour is observed. (C) 2003 Elsevier Science B.V. All rights reserved.