A study has been made of the direct oxidation of ethylene to ethylene oxide in a flowing system under the catalytic action of silver. Various catalysts were prepared and one containing 10 per cent. barium carbonate, having a high and sustained activity, was selected for a detailed examination of the influence of reaction conditions on the yield of ethylene oxide. It has been found that the percentage ethylene in the reaction mixture can be varied widely without greatly changing the yield of ethylene oxide, but that the conversion is decreased by increasing the ethylene content. The yield increases with increasing oxygen content until the latter reaches 20 per cent. after which further enrichment with oxygen has little effect. The presence of water vapour slightly increases the yield, hut considerably decreases conversion. The yield is much decreased by diminishing the reaction pressure. Highest yields (73 per cent.) were obtained when small amounts of ethylene dichloride were added to the reactants but at the expense of low conversion. Propylene and isobutylene yielded only carbon dioxide and water when oxidized in the presence of a silver catalyst. The results are discussed in relation to the reaction mechanism suggested by Twigg(1).