The ethylene epoxidation over Ag catalysts is one of the most important industrial processes converting several billion US dollars annually due to the importance of ethylene oxide (EO) as a versatile chemical intermediate. Despite the intensive application of this process, the reaction mechanism is still under debate in the literature. One reason for the lack of detailed understanding of the heterogeneous catalytic reaction is the existence of many different oxygen species on the silver catalyst surface. The application of new in-situ methods like ambient-pressure X-ray photoelectron spectroscopy allows investigation of the role of the different oxygen species in the ethylene epoxidation reaction. The formation and transformation of the oxygen species and particle-size effects will be discussed in terms of the catalytic reaction.