Several nanosized (less than 5 nm) metal peroxide nanocrystals, e.g., ZnO2, CaO2, MgO2, and BaO2, were successfully synthesized by a generic and facile one-step liquid-phase precipitation route at room temperature. These peroxides possess fascinating capabilities for oxygen storage, delivery, and generation when they are hydrolyzed or pyrolyzed in aqueous or hot environment. Oleic acid capping agents were used in the preparation to mitigate aggregation of the particles. Both the oxidation state of each element and the stoichiometry of each compound were unchanged during the process. The ultrafine particles and well-developed crystallinity suggest a `bottom-up¿ or direct peroxide ion (O2− 2 ) mediated reaction in solution. The ability of these compounds for rapid release of oxygen indicates that these materials are good candidates for applications in biometabolic and aerobic reaction chemistry.