The species formed after adsorption of NO on a Cr-ZSM-5 sample pre-subjected to different red-ox treatments have been studied by FTIR spectroscopy. Only NO+, occupying cationic positions in the zeolite, is formed on a non-reduced sample. Adsorption of NO on samples reduced by hydrogen at temperatures between 473 and 773 K leads mainly to formation of different mononitrosyl (1890 and 1815 cm(-1)) and dinitrosyl species (1910 + 1780 cm(-1)). The latter are decomposed without producing a measurable fraction of the corresponding mononitrosyls. The dinitrosyl complexes are likely produced with the participation of Cr3+ cations. Some Cr2+ sites are created after reduction of the sample with hydrogen at 873 K or with CO at 773 K. These sites form another kind of dinitrosyl species (bands at 1902 and 1768 cm(-1)) which are converted during evacuation to mononitrosyls (band at 1782 cm(-1)). The assignments of the different bands are supported by (NO)-N-14-(NO)-N-15 co-adsorption experiments. (c) 2006 Elsevier B.V. All rights reserved.