Adsorption of NO on oxidized WO3-ZrO2 catalysts leads to formation of adsorbed N2O, surface nitrates, NO+, and Zr4+(NO3-)-NO species. When NO is adsorbed on a sample reduced at 523 K, W5+-NO (1855 cm(-1)) and W4+(NO)(2) (1785 and 1700 cm(-1)) species are formed in low concentration. Reduction at 573 K increases the density of W4+ sites and this density remains unchanged after reduction up to 673 K. The W4+(NO)(2) species are stable towards evacuation and in the presence of oxygen; however, they quickly disappear in the simultaneous presence of NO and O-2 as a result of the oxidation of the W4+ cations to W6+. The density of the W4+ sites on a Pt/WO3-ZrO2 sample is significant even after reduction at 523 K. This is explained by the promotion effect of platinum on the support reduction. The use of NO as a probe molecule for detection of reduced Wn+ sites on tungstated zirconia is discussed.