The modification of clean silver foil by C2H4 + O2 and CO + O2 reaction mixtures has been studied by XPS, TPD, and TPR. It has been shown that C2H4 + O2 treatment activates the clean silver surface for ethylene oxide formation. This is accompanied by the formation of a number of surface species (carbonates, elementary carbon and adsorbed oxygen), as well as the dissolution of oxygen and carbon atoms in the silver bulk. Displacement of ethylene in the reaction mixture by CO and comparison of composition of the silver surface after action of both mixtures indicates that the surface active centre contains two adsorbed oxygen states with different ionicity of the Ag-O bond. "Ionic" oxygen [E(b)(O 1s) = 528.4 eV] produces Ag+ ions as sites for ethylene adsorption, while "covalent" oxygen [E(b)(O 1s) = 530.5 eV] seems to react with the adsorbed ethylene to form ethylene oxide. A possible mechanism of the "covalent" oxygen formation and influence of different factors (surface composition, morphology, etc.) on this process are discussed.