Fourier transform IR spectra of carbon monoxide adsorbed at liquid nitrogen temperature on a series of alkalimetal exchanged ZSM-5 zeolites [X-ZSM-5 (X = Li, Na, K, Rb, Ca)] show that the dipolar:CO molecule specifically interacts with the zeolite extraframework metal ions. This interaction modifies the C-O stretching frequency, which increases continuously from 2157 cm(-1) for Cs-ZSM-5 upto 2188 cm(-1) for Li-ZSM-5. It is shown that the observed frequency shift (from nu(CO) = 2143 cm(-1)) can be used to evaluate the electrostatic field created by the metal cations. Nonspecifically adsorbed (physisorbed) CO was also observed as well as some minor IR absorption bands which were tentatively assigned.