Optimal ethylene oxide selectivity during steady-state ethylene oxidation over Cs and Re-promoted Ag/alpha-Al2O3 catalysts is attained only if co-fed alkyl chlorides deposit chlorine adatoms and co-fed alkanes moderate surface chlorine coverages. Surface chlorine coverage under reaction conditions was measured in chemical transient studies immediately after reaching steady-state by removing reactants in flowing helium and recirculating ethane and oxygen in a gas-phase batch reactor; this resulted in removal of chlorine from the catalyst as ethyl chloride and enabled assessment of the total number of moles of chlorine desorbed per surface silver atom, which varied from 0.10 to 0.45 depending on the process conditions employed. Batch recirculation experiments demonstrated that oxygen is required to remove chlorine from silver catalysts, suggesting molecular events akin to those involved in oxychlorination of alkanes moderate chlorine coverage during steady-state catalysis. The chlorine content was largely unperturbed by changes in ethylene (20-40 mol%), carbon dioxide (1-5%), or oxygen (2-7%) concentrations during steady-state catalysis. Increasing temperature resulted in a monotonic decrease in chlorine coverage. A power law model accurately describes trends in chlorine coverage as a function of steady-state ethane (0.1-15 mol%) or ethyl chloride (0.5-6.3 ppmv) concentrations. The carbon selectivity for ethylene oxide increases with chlorine coverage, in agreement with expectations from steady-state measurement of rates and selectivity as a function of co-fed alkyl chloride promoters. These results explicate the critical role of oxygen-containing species in moderating Cl coverage during ethylene epoxidation catalysis, include a model that describes chlorine coverage across a broad concentration range of alkyl chloride promoter and alkane moderators, and enable correlation between kinetic parameters measured as a function of process conditions and the predicted chlorine coverage. (C) 2018 Elsevier Inc. All rights reserved.