ABSTRACT:The formation of highly reactive oxygen species (ROS) on metal oxide surfaces have attracted considerable interest due to their diverse applications. In this work, we have performed density-functional theory calculations to investigate the co-adsorption of oxygen and water on ultrathin MgO(100) films deposited on Mo(100) substrate. We reveal that the molecular oxygen can be stepwise decomposed completely with the assistance of water. Consequently, a series of highly ROS including superoxide, hydroperoxide, hydroxyl and single oxygen adatom are formed on Mo(100) supported MgO(100) thinfilms. The reaction barriers accompanied by the generation of ROS are reported, and the influence of the thickness of MgO(100) films is also discussed. The most promising routes to produce these fascinating species provide valuable information to understand the importance of synergistic effect, namely the substrate, the co-adorbed species, and the film thickness, in multiphase catalyst design.Key words: reactive oxygen species adatom, hydroxyl, hydroperoxide, DFT, interface Reactive oxygen species (ROS) play essential roles in chemical and biological processes. Understanding the mechanisms of how to generate ROS on metal oxide surfaces is of fundamental interest, as we can selectively control the chemical reactions if we know how to generate ROS. Recently, there has been a surge in researches pertaining to the physical and chemical properties of multiphase interfaces. 1-2 These interactions between solid, liquid, and gas play a key role in solv-