The adsorption of NO2 was investigated and compared on Na- and Ba-Y, FAU zeolites both in the absence and presence of adsorbed water using FTIR and TPD techniques. The same ionic NOx species (NO+, NO+ -NO2, NO3-, formed by the disproportionation of NO2, were observed to form on both materials under dry conditions at room temperature. The thermal stabilities of these species, however, were vastly different on the two materials. Room-temperature evacuation was sufficient to decompose the NO+NO2 adduct in Na-Y, while this species was stable up to 350K over Ba-Y The adsorbed NO+ was also much more stable over Ba-Y than on Na-Y. Water significantly affected the adsorbed NO, species on both materials. In the presence of water, the IR signatures of adsorbed NO+ were eliminated from both catalysts; however, it did not affect the IR feature of the NO+NO2 species on Ba-Y. In the TPD spectra, the NO2 desorption peak shifted from 350 K to 520 K on Na-Y preexposed to water. In Ba-Y, the high-temperature NO2 desorption feature of similar to470 K shifted to similar to620 K as a result of adsorption on the water-containing sample, while the low-temperature peak remained unchanged.