The interaction between NO and the heteropolytungstic acids (HPAs) H3PW12O40 (HPW), H0.5Cs2.5PW12O40 (HCsPW), HMgPW12O40 (HMgPW), H8SiW11O38 (HSiW11), H4SiW12O40 (HSiW), and H10CoW12O42 (HCoW) in the presence of O-2(g) was investigated. The tools employed were in situ diffuse reflectance infrared spectroscopy, X-ray powder diffraction, and solid-state H-1 NMR. It was determined that protons may either be present in the HPAs secondary structure as anhydrous protons or be bound to one or two water molecules as H3O+ or H5O2+, respectively. A previous investigation found that HPW sorbed NO into its bulk structure as NOH+, whereas the anhydrous potassium salt of HPW exhibited weak chemisorption of NO on its surface. In the present study, it was found that NO chemisorbed weakly on the surface of the anhydrous HCsPW. For HMgPW and HSiW, IR and NMR evidence suggests that water is present as H3O+, and the formation of surface-bound NOH+ was observed. Inclusion of NO into the secondary structure as NOH+ was observed for HPW, HSiW11, and HCoW. Literature data for HPW, as well as IR and NMR results reported here, indicate that these HPAs contain water as H5O2+ The presence of H5O2+ is, therefore, a prerequisite for NO incorporation as NOH+ in the HPA secondary structure. These HPAs exhibited two modes of NO bonding in the secondary structure: a strongly hydrogen-bound hydrated form, NOH+. H2O. and a more weakly bound anhydrous form, NOH+. Evidence suggests that it is the hydrated form that decomposes, yielding N-2 upon rapid heating. Both NO-containing species interact with terminal and corner-sharing oxygen atoms of the Keggin ion. Anions that are held apart by terminal oxygen-hydrogen bonds have a larger lattice parameter than those held apart by terminal oxygen-hydrogen and corner-sharing oxygen-nitrogen bonds. The lattice contraction upon NO incorporation implies activation of NO by an intimate interaction with oxygen of the Keggin anion.