Metal peroxides (MO₂) have long been used as preferred reagents in organic and inorganic reactions as well as in plastic processing and concrete industries due to the characteristic O–O peroxo linkage that is easily cleaved at moderate temperatures to produce activated oxygen and metal oxide. While these compounds are usually obtained by reacting metal ions with H₂O₂ under basic conditions at elevated temperatures, we demonstrate that the nanoparticles of MgO₂, ZnO₂ and CdO₂ can be accomplished by reacting the corresponding metal oxides with H₂O₂ under ambient conditions. These peroxide nanocrystals exhibit the lowest decomposition temperatures (T_d), 25–80 °C lower than the reported values, making them suitable for solution based reactions. It is found that the lowering of T_d can be controlled by tailoring the metal-oxide defects induced by the synthesis procedure. Based on a similar reaction mechanism, we now demonstrate that ZnS as well as Zn can also be converted to ZnO₂ nanocrystals under ambient conditions. The possibility of converting metal oxides to metal peroxides and vice versa makes it a recyclable process and the lower T_d is expected to expand their usage in solution processes.