The low temperature adsorption of CO and the room temperature adsorption of propionitrile (PrN), isobutyronitrile (IBN) and pivalonitrile (2,2dimethyl-propionitrile, PN) have been investigated on NaX (Si/Al atomic ratio = 1.3) and NaY (Si/Al atomic ratio = 2.4) zeolites by FIF-IR spectroscopy. The bands of CO adsorbed on NaY have been reassigned. The relevance of Na+ ions at S-III or S-III' positions also on NaY has been emphasized. Evidence is provided for the formation of complex interactions where nitrile molecules interact with more than one site, likely more than one Na ion, as previously observed on alkali metal containing MOR zeolites. The possible relevance of this interaction is briefly discussed. (c) 2006 Elsevier B.V. All rights reserved.