The chemical reactions in the system O-2-NOx has been studied in a conventional furnace. The reaction rates of formation and decomposition of NOx have been measured by monitoring the isotopic concentration in a closed system (conventional furnace) and using isotopical labeled (with N-14, N-15, N-16, O-16, and O-18) NOx and O-2 gas mixtures. It was found that O-2 and NO reacted at mom temperature to NO2 and N2O. The oxygen in NO2 came almost exclusively from NO and the oxygen in N2O fame almost exclusively from O-2 The reaction probably occurs on the walls of the system and a considerable amount of NO and O-2 is adsorbed on the walls of the system at room temperature. This is seen in O-2 and NO mixtures by a decrease of pressure in the system with time. Low temperatures favor the formation of NOz and high temperatures result in decomposition of both NO2 and N2O.