The effect of charge balancing cations on encapsulation of CO in the pores of Y-zeolite has been investigated using FTIR spectroscopy. The multiple vibrational bands in CO and CO stretching regions, originating from the clusters of these molecules and developed on exposure of CO on Na-Y, undergo an identical shift in frequency and relative intensity when Na+ are exchanged with group IIA cations. Thus, a constant blue shift of similar to 8, 12, and 6 cm(-1) was observed in all the nu(CO) bands for the samples exchanged with Ca2+, Sr2+, and Ba2+, respectively. The respective Delta nu(CO2) on Ca2+, Sr2+, and Ba2+ exchanged zeolite-Y were found to be around +6, +2, and -2 cm(-1). The observed frequencies of all the CO and CO2 vibrational bands and their respective Delta nu values with respect to gas-phase frequency are found to show a close relationship with the ionic radius, pore volume, electrostatic potential (e/r), or the field strength (V/Angstrom) associated with the exchanging cations. The data reveal that the characteristics of zeolitic pores play a more important role in encapsulation of CO molecules, rather than the direct bonding of CO at cation sites.