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 oxide becomes possible. Calculated transition state theory rate constants for the elementary steps involved in the reaction predict that the selectivity of Au(111) toward epoxide is of similar to 40% in good agreement with recent experimental findings for styrene epoxidation on Au(111).