A complementary experimental and theoretical study of the alloying effects of Sn on the molecule-surface bonding of crotonaldehyde on Pt(111) is presented. By comparing high-resolution electron energy loss spectroscopy (HREELS) data and density-functional theory (DFT) calculations of vibrational spectra from a complete set of possible adsorption configurations of crotonaldehyde on the Pt(111) and Pt Sn(111) surfaces, we are able to identify the adsorption structures present on the surface. An interpretation of the HREELS spectra for these highly complex systems is proposed and the effect of alloying with Sn on the molecule-surface interactions is addressed. (c) 2006 Elsevier B.V. All rights reserved.