Partial oxidation of ethylene with silver catalysts is an exothermic reaction:

It is normally accompanied by two even more exothermic side or secondary reactions. These are the complete combustion of ethylene, which is the main source of CO₂, and the further oxidation of ethylene oxide:

Industrial processes attain a selectivity of 65-75% ethylene oxide (air process), or 70-80% (O₂ process), with a total heat of reaction of 85-130 kcal(357-546 kJ)/mol ethylene. According to the reaction mechanism, the maximum possible ethylene oxide selectivity is only 80%.

The specific O₂ activation on the metal surface of the silver is the fundamental reason for its catalytic acitivity. Initially, oxygen is adsorbed molecularly on the silver and reacts in this form with ethylene to yield ethylene oxide. The atomic oxygen generated cannot produce more ethylene oxide; it oxidizes ethylene or ethylene oxide to CO and H₂O:

Industrial catalysts generally contain up to 15 wt% Ag as a finely divided layer on a support. All other specific catalyst characteristics are proprietary and, together with technological differences, account for the various processes in existence today. The activity and selectivity of the catalyst is primarily influenced by the manufacturing process, the type of support and its physical properties, and any promoters or activators.

In all processes, inhibitors are used to prevent total oxidation. Several ppm of 1,2-dichIoroethane are often introduced to the reaction mixture for this purpose. The chemiabsorbed atomic chlorine - from the oxidation of hydrogen chloride obtained in the dehydrochlorination of 1,2-dichloroethane - hinders the dissociative chemiabsorption of atomic oxygen and therefore the combustion of ethylene to CO₂ and H₂O.