An improved process for the manufacture of ethylene oxide through the epoxidation of ethylene using a catalyst comprising silver and at least one efficiency-enhancing salt of a member of a redox-half reaction pair. Added to the epoxidation reaction is a two-component gas-phase promoter system comprising a chlorine-containing component (for example ethyl chloride, methyl chloride, vinyl chloride and ethylene dichloride), and a nitrogen-containing component of nitric oxide and other compounds capable of generating under reaction conditions at least one gaseous efficiency-enhancing member of a redox-half reaction pair comprising NO, NO.sub.2, N.sub.2O.sub.3 or N.sub.2O.sub.4. The amount of each component of said gaseous promoter is adjusted to maintain the ration of N* to Z* less than or equal to 1 wherein, N* is the nitric oxide equivalent in ppmv, ranging from 1 to 20 ppmv and Z*=ethyl chloride equivalent (ppmv)*100 percent/ethane equivalent (mol percent)*100 ranging from 5 to 40 ppmv.