The reaction intermediates adsorbed on Cu2+-exchanged ZSM-5 were studied for the reduction of nitric oxide with ammonia in the presence of oxygen. IR measurements using isotopes of O-18(2) and (NO)-N-15 revealed that a nitrate species was formed on Cu-ZSM-5 having preadsorbed ammonia to give bands at 932 and 1630 cm(-1) and reacted with the adsorbed ammonia species to form nitrogen at 573 K. It was proposed that the cm nitrate species is the reaction intermediate for the reduction of nitric oxide with ammonia.