The effect of co-adsorbed Cs and Cl on the selectivity of ethylene epoxidation was studied computationally on an Ag2O(0 0 1) surface model. Cl blocks the oxygen vacant sites and prevents oxometallacycle (OMC) formation, which is responsible for aldehyde formation on the oxide surface. Cl also prevents unfavorable surface reconstruction induced by co-adsorption of ethylene. It also has a positive effect on the stability of selective oxygen species when Cs co-exists. Introduction of Cs on the Ag2O(0 0 1) surface results in the formation of CsOx-type of complexes in the absence of Cl. Whereas oxygen atom adsorption energies are increased, Cs also introduces surface oxygen vacancies that reduce catalyst selectivity. Cs and Cl co-exist on the surface in the form of CsClxOy complexes. Under reaction conditions, these complexes stabilize the oxide surface and reduce oxygen surface vacancy formation, which results in higher ethylene oxide (EO) selectivity.