A series of Cs-Ag, Re-Ag, Cs-Re-Ag, and Cs-Re-Mo-Ag (supported on alpha-Al2O3) catalysts has been prepared and evaluated for ethylene epoxidation to determine the mechanism of selectivity enhancement for high-selectivity ethylene oxide (EO) catalysts which contain high-valent Re oxyanions and other copromoters, such as MoO42-. Optimal amounts of Re (as NH4ReO4) and Mo [as (NH4)(2)MoO4] on Cs-promoted, Ag/alpha-Al2O3 increase EO selectivity from approximately 79% to 83%. Analyses by XPS and SEM suggest the origin of both Re and Cs promotion is electronic. SEM shows that neither Re nor Mo changes the morphology or particle sizes of the Ag particles. XPS analyses show that Re shifts Ag 3d BE to higher values, which enhances electrophilic attack by oxygen adsorbed on Ag (Ag-O) at the electron-rich C=C bond of C2H4. In the subsequent step, Cs lowers the Ag 3d BE to facilitate desorption of the EO precursor to form gas phase EO. That is, Re and Cs promote different steps of the mechanism of EO formation. A reaction scheme detailing the mechanism of EO formation over Re-modified, Cs-promoted Ag catalysts is presented. (C) 2014 Elsevier Inc. All rights reserved.