Direct synthesis of hydrogen peroxide (H2O2) from H-2 and O-2 is an ideal route. H-2/O-2 plasma has a great potential for direct synthesis of high purity H2O2 without purification operations. However, low yield and high energy consumption limits the application of H-2/O-2 plasma in industry. This article reports that gas state Ar and H2O molecule serving as molecular catalysts promoted the synthesis of H2O2 from H-2/O-2/Ar/H2O plasma dramatically: the H2O2 yield was enhanced by 244% and the energy consumption was reduced by 70.9%. Ar not only increased the electron density, but also selectively accelerated the dissociation of H-2 toward the formation of center dot HO2, a key intermediate species in H2O2 synthesis. While H2O facilitated the formation of center dot HO2 radical and stabilized it by forming a HO2 center dot H2O complex, resulting in enhancing the H2O2 production. This single molecular catalysis reduced the cost of H2O2 synthesis more than 50%. (C) 2017 American Institute of Chemical Engineers