Adsorption of NO and its coadsorption with oxygen on WO3-ZrO2 as well as the reactivity of the surface NOx species toward propene have been studied by FTIR spectroscopy and temperature-programmed desorption. NO adsorption on WO3-ZrO2 results in the formation of N2O (2282, 2243, and 1224 cm(-1)), NO+ (2140 and 2119 cm(-1)), surface nitrates (1614, 1570, and 1230 cm(-1)), and nitrosyls (1936 cm(-1)) created on cationic sites, which are affected by nitrates. Except for N2O, the other species are developed with time. Addition of oxygen to the NO-WO3-ZrO2 system results mainly in an increase in the concentration of the surface nitrates. The latter are stable up to ca. 673 K and are thermally decomposed, yielding NO and NO2, at higher temperatures. The nitrates interact with propene at ca. 473 K, giving an organic deposit which is decomposed to isocyanates (2296, 2278, and 2250 cm-1). These species easily react with a NO + O-2 mixture, producing dinitrogen. The assignments of the IR bands are supported by N-15-labeling experiments and analysis of the spectral region where the overtones and combination modes of surface nitrites and nitrates appear. Finally, some new aspects of the mechanism of selective catalytic reduction of NOx by hydrocarbons are discussed.