An extensive diffuse reflectance infrared investigation of the interaction of 12- tungstophosphoric acid (HPW) and its potassium salt with NOx (x = 1 or 2) was performed. In the presence of KBr, HPW yielded a spectrum identical to that of the potassium salt. Using pure powders, spectra of HPW . 6H(2)O showing the (O-H) bands of H5O2+ and the correct appearance of the Keggin anion bands are presented. At 423 K, NO(g) with or without O-2(g) interacted weakly or not at all with the potassium salt; however, NO2(g) adsorbed onto the surface as loosely bound NO with v(NO) at 1870 cm(-1). The hexahydrate did not sorb NO in the absence of oxygen at 423 K or higher. In the presence of oxygen at 423 K, the surface of HPW is involved in catalytic oxidation of NO(g) to NO2(g) via an adsorbed NOxy- intermediate. This species can be absorbed into the HPW structure as a form of NOH+, displacing water, or leave as NO2(g). Not all structural water is displaced by NO, and water plays a role in holding the new structures together by hydrogen bonding. The v(NO) band of absorbed NOH+ consists of two overlapping bands, one at 2267 cm(-1), assigned to NOH+. H2O, and a second broader band at 2260 cm(-1), assigned to NOH+. Both forms bridge between the O-t and O-c of two Keggin ions. It is proposed that NO first displaces a H2O to give the NOH+. H2O moiety, causing rearrangement of the hydrogen bound secondary structure. This structural contraction squeezes water out of adjacent voids, leading to formation of the sorbed anhydrous NOH+ moiety. In HPW, NO displaces water at a particular temperature until a stable configuration in the secondary structure is reached. The temperature and amount of water present determine the relative amounts of H5O2+, H3O+, NOH+. H2O, and NOH+ present.