By increasing the ethylene concentration, the chemical system was more favourable towards the partial oxidation of ethylene. The oxygen concentration had a positive effect on the conversion and selectivity towards ethylene oxide. A clearly improved catalyst stability and activity were achieved by pretreating the silver microplates during 12 h with ethylene. The reaction order was close to one with respect to oxygen, while the reaction order of ethylene was around 0.5. A new generalized kinetic model was proposed for ethylene oxide formation. The model is consistent with the experimental data, predicting correctly the observed reaction orders and selectivities. The work demonstrated that gas-phase microreactors combined to micro gas chromatography are strong tools in kinetic research.