Oxygen adsorption on supported silver depending on the cluster sizes is studied by XPS and TPD using a model carbon-supported silver catalyst. It has been shown that the electrophylic oxygen active in ethylene epoxidation is produced on the small silver clusters (less than or equal to 100 Angstrom) more effectively than on the bulk metal, most probably, due to formation of subsurface oxygen. The desorption of the electrophylic oxygen at 450 K proves the thermodynamic possibility of its participation into ethylene oxide formation. Enlargement of the silver particles decreases the amount of subsurface oxygen and results in the appearance of nucleophylic oxygen. These results are used for discussion of the possible explanation of the size effect in ethylene epoxidation over Ag/Al2O3 catalysts.