Two Y zeolites (FAU) used in the formulation of commercial FCC catalysts were studied with progressive CO adsorption by FTIR at 77 K. We investigated changes in the type, concentration, and strength of the acid sites when the zeolites were deactivated under conditions that simulate the regenerator of a commercial FCC unit. Y zeolites treated with steam and subsequent removal of the extralattice aluminum species showed pores and defects in structure allowing diffusion of CO molecules toward low-frequency (LF)OH groups in the beta cavity, while in zeolites with plenty of extraframework aluminum, these sites were not accessible. Strong Bronsted acid sites were found with the position of a perturbed -OH band as low as 3180 cm(-1), a nu(CO) at 2183 cm(-1), a DeltaOH of 443 cm(-1), and a proton affinity (PA) of 1084 kJ mol(-1). Hydrothermal treatment at 1061 K in an air-steam mixture destroyed the zeolite acid sites, including the extraframework aluminum -OH groups. Knowledge about the changes in the acidity of the zeolite and the other FCC catalyst components under conditions that simulate deactivation in a FCC regenerator unit will improve the selection and evaluation of catalysts for commercial units.