A detailed theoretical study is presented focusing on the role of halogens as promoters for the silver-catalyzed partial oxidation of ethylene to the epoxide (EO) and, in particular, aiming to understand why Cl is best. The study is based on periodic density functional theory calculations carried out for the reaction between ethene and atomic oxygen on the Ag(111) surface, taken as a model catalyst, in the presence of coadsorbed halogens. It is found that the presence of co-adsorbed halogens significantly decreases the energy barrier from the oxametallacycle intermediate (OMME) to EO relative to that from the same OMME intermediate to the undesired acetaldehyde (AC) product. However, co-adsorbed halogens are also found to increase the desorption energy of the reaction products. We present compelling evidence that the superior activity of Cl is due to the additional presence of subsurface Cl that favors EO formation with respect to AC but at the same time facilitates EO desorption, hampering further transformations. The results provide a signpost for the rational design of catalyst promoters. (C) 2008 Elsevier Inc. All rights reserved.