SO2 adsorption and desorption on NaX and NaY zeolites, as a function of contact time and temperature, were studied combining in situ FTIR (diffuse reflectance with controlled chamber) and thermogravimetry. It is shown that SO2 is physisorbed as a liquid on both zeolites at room temperature. Furthermore, SO2 is chemisorbed on NaX, giving HSO3-, which is transformed during the desorption treatmentinto S2O52-. The presence of this species in this system has never been observed before by IR spectroscopy. Additionally, the geometry of the adsorbed species is discussed in detail. The SO2 retention capacities of the zeolites was assessed by thermogravimetry and related to their porous structure. The different Si/Al ratios of both isostructure zeolites were found to be responsible for their different behaviors.