Infrared spectroscopy has been applied to the study of NO adsorption, NO + O-2) coadsorption and interaction of surface nitrates with methane on four samples: ZrO2, SO42- -ZrO2), Co/ZrO2 and Co/SO4 (2-)-ZrO2. Adsorption of NO on zirconia is negligible, while nitrosyls of the Zr4+ (SO42-)-NO type are formed on SO42--ZrO2. Adsorption of NO on Co/ZrO2 results mainly in formation of cobalt mononitrosyls (band at 1872 cm(-1)) and dinitrosyls (1855 and 1777 cm(-1)), which are stable during evacuation at room temperature. The same types of compounds are also formed after NO adsorption on Co/SO42- -ZrO2. However, in this case the mononitrosyls are characterized by a band at 1922 cm(-1) whereas the dinitrosyls have bands at 1903 and 1814 cm(-1). Both types of nitrosyls decompose slowly during evacuation. Addition of oxygen to the NO-sample systems causes decomposition of the nitrosyls and appearance of various kinds of surface nitrates, part of them formed on the support. The nitrates produced with the participation of cobalt ions react with methane at relatively low temperatures (about 473 K), which indicates that they are intermediate compounds during selective catalytic reduction (SCR) of NOx with methane. The effect of preliminary sulfatation of zirconia on the properties of deposited cobalt cations has been discussed. (C) 2002 Elsevier Science B.V All rights reserved.