Whereas NO adsorption at room temperature on activated H-ZSM-5 (Si/Al = 29) caused only negligible changes in its IR spectrum, addition on O-2 to NO led to the appearance of bands at 2133 and 977 cm(-1). Concomitantly, the number of acidic zeolite OH groups decreased while H2O hydrogen-bonded to zeolite OH groups developed. Introduction of small amounts of O-18(2) did not change the 2133 cm(-1) band wavenumber, nor the use of a partly deuteroxylated D-H-ZSM-5 sample. In such a case, HOD formation was detected. The results obtained evidence that the 2133 cm(-1) band, generally considered as characterizing NO2+ species, is, in fact, due to NO+ species occupying cationic positions in the zeolite. The 977 cm(-1) band is attributed to the O-lattice-NO+ vibration. A scheme of the NO+ formation, involving NO2 molecules as NO oxidizing agent, is proposed.