The active oxygen species for ethylene epoxidation on Ag surfaces has been a long-standing issue. We conducted in situ observations of Ag(111) surfaces under exposure to ethylene and oxygen gases with near ambient pressure (NAP) X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. Combined results of these spectroscopies evidence that a carbonate species with a flat-lying configuration is formed on the Ag(111) surface at 370-500K under exposure to the NAP gas mixture of ethylene and oxygen.