This work presents a periodic density functional study of the epoxidation mechanism of ethylene on Au(111). It is found that, once atomic oxygen is adsorbed on the surface, partial oxidation to ethylene oxidebecomes possible. Calculated transition state theory rate constants for the elementary steps involved in thereaction predict that the selectivity of Au(111) toward epoxide is of ∼40% in good agreement with recentexperimental findings for styrene epoxidation on Au(111).