The adsorption and reaction of trans-1,2-dichloroethene on CuPd(110) have been studied using molecular beam adsorption reaction, temperature-programmed desorption, reflection absorption infrared, high-resolution electron energy loss, and X-ray photoelectron spectroscopies. Below 165 K, the molecules adsorb intact at all coverages with their molecular planes orientated largely parallel to the metal surface. Above 165 K, decomposition is observed initially on adsorption, but is limited by availability of surface sites and stablisation effects of coadsorbed Cl. Between 300 and 350 K the main desorbing decomposition products are H-2 and C2H2, while only H-2 is produced between 350 and 675 K. Above 675 K evolution of HCl is observed. The decomposition of the hydrocarbon skeleton occurs mainly at Pd sites, while dechlorination is catalysed at Cu sites with Cl preferentially binding to the Cu atoms. This leads to dynamic changes in surface composition of the alloy with segregation of underlying Cu atoms to the surface that scavenge the Cl atoms and thus help to keep the Pd sites Cl free. Crown Copyright (C) 2004 Published by Elsevier Inc. All rights reserved.