The absolute number of Lewis sites has been semiquantitatively estimated by measuring the integrated absorbance of CO adsorbed at similar to 100 K on aluminas and dealuminated mordenites. This has been made possible by using the known amount of Bronsted sites in zeolites and the linear regression between the specific absorbance and the frequency. This linear regression as well as the linear regression between the CO adsorption energy and the frequency has been justified on the basis of the perturbation of an anharmonic oscillator by an electric field. From the absolute number of Lewis sites it is concluded that the nonframework aluminum debris in dealuminated mordenite are forming nanosized particles in the zeolite pores. Two strong Lewis sites are observed on the surface of these particles as well as on the surface of aluminas. A possible assignment is suggested.