The reactions of ethylene oxide with Hg(3P1) atoms have been investigated at 25°C at pressures between 10- and 600-mm Hg, and at 200°C and 300°C at 100-mm Hg. The principal products are H2, CO, CH3CHO and a polymer; smaller amounts of CH4, C2 hydrocarbons and formaldehyde are also formed. The nature of the products and the very small effect of pressure on the reaction are best explained by a mechanism based principally on the formation of the activated ethylene oxide molecule, with a minor portion of the reaction proceeding via a C–H split.