NO+ nitrosonium species have been evidenced by in situ FTIR spectroscopy, on H-ZSM-5 and H-Cu-ZSM-5 zeolites under NO+O-2 flow (near reaction conditions) at 623 K. Propane introduction in the reaction stream leads to NO+ disappearing and to acrylonitrile detection among reaction products on H-ZSM-5. In the presence of copper, acrylonitrile adsorbed on Cu+ is rapidly decomposed into ethylene and cyanide Cu+CN in these conditions. In a further step, cyanide isomerises into isocyanide Cu+NC. A reaction pathway involving H+ and Cu+ zeolitic sites is proposed for isocyanide species formation from reactants. The transformation of isocyanide species into N-2 (already published) is recalled in order to define a complete mechanism for NOx selective catalytic reduction.