The low temperature adsorption of CO and the room temperature adsorption of acetonitrile, propionitrile. isobutyronitrile, pivalonitrile, benzonitrile, and o-toluonitrile on Na-mordenite (NaMOR) have been investigated by Fourier transform infrared (FT-IR) spectroscopy. The results have been compared with analogous experiments performed on H-mordenite. Na-X zeolite. and Na-silica-alumina. The Na distribution in NaMOR has also been investigated by X-ray diffraction and far-IR spectroscopy. The conclusions are that Na+ ion distribution is essentially random and that, together with the well-known interaction of the probes with Na+ ions in the side pockets and the main channels, a stronger additional interaction occurs in all cases. This new interaction is likely multiple, involving either more Na-divided by ions or Na and oxygen species. This interaction is more pronounced with the hindered nitriles. whose access at the main channels is likely forbidden. This suggests that this interaction, which is also observed on Na-X zeolites but not with Na-silica-alumina. occurs at the external mouths of the mordenite channels.