The C-O stretching frequencies, obtained by a 77 K Fourier transform infrared study of the interaction of CO with the extraframework cations in alkali-metal exchanged M-ZSM-5 and M-mordenites (M=Na,K,Rb,Cs) have been used to evaluate the local electric field strength tested by the probe molecule. For all the investigated zeolites, the dipolar CO molecule was found to interact specifically with extra-framework metal ions, and its stretching frequency was found to increase continuously from Cs+ to Na+ samples. The observed frequency shifts, with respect to that of the free CO molecule (($) over bar nu(CO)=2143 cm(-1)), were used to evaluate the electrostatic field created by the cation sites. Electric fields in the range of 2 to 6 V nm(-1) were obtained; experimental values are inferior to those expected if generated in vacuo, at the distance of the probe molecule, from a point charge of +e The observed fields can be reproduced by the sum of a positive contribution of the cation itself, and by two negative contributions, the first due to the negatively charged oxygen atoms surrounding the cation and the second due to the polarized zeolite framework. The last contribution has been found to increase from ZSM-5 to Mordenite, in agreement with the decreasing Si/A1 ratio.