This paper reports isotopic evidence on non-thermal plasma-induced fixation of gas-phase oxygen on the surface of several catalysts such as TiO2, Ag/TiO2, Ag/gamma-Al2O3 and Ag/MS-13X at atmospheric-pressure. On-line mass spectrometric analysis and stoichiometric comparison of reactants and products revealed that the fixed surface oxygen can be activated by nonthermal plasma. The fixed O-18 by nonthermal plasma survived for a certain period of time (about 30 min), and involved in the formation of isotope-exchanged oxygen ((OO)-O-18-O-16) and isotope containing COx (CO and CO2).