The nature of the NO, species produced during the adsorption of NO at room temperature and during its coadsorption with oxygen on pure and sulfated zirconia has been investigated by means of in situ FFIR spectroscopy. The adsorption of NO on both samples occurs through disproportionation leading to the formation of nitrous acid; water molecules; nitro species; and anionic nitrosyls, NO-. A mechanism for the formation of these adsorption forms is proposed. The NO- species are stable on the surface of zirconia, whereas on the sulfated sample, they are readily oxidized by the SO42- groups. The process of NO disproportionation is favored by wet surfaces and occurs with participation of the tribridged (ZrO2) and terminal (ZO(2)-SO42-) hydroxyl groups. Coadsorption of NO and O-2 on pure zirconia leads to the formation of various kinds of nitrate species. The presence of sulfate ions reduces the amount of surface nitrates and decreases their thermal stability. An analysis of the combination bands of the nitrate species shows that this spectral region can be used for structural identification of bidentate and bridged nitrates.