Silica supported 12-tungstophosphoric acid (H3PW12O40 Or HPW), as well as unsupported HPW, were examined for NO absorption and temperature programmed desorption by in situ diffuse reflectance infrared spectroscopy and TPD-MS. The SiO2 supported samples contained HPW at 2, 4, and 8 wt% W loading levels. X-ray powder diffraction for determination of HPW lattice parameter and crystal size, solid state proton NMR, and ex situ IR were also employed to characterize these catalysts. The results indicate a strong interaction between HPW and the surface hydroxyl groups of the silica support, leading to the formation of an entirely different secondary structure than observed in bulk HPW. In this secondary structure, water is proposed to be present at H3O+ rather than H5O2+ as in unsupported HPW. This structure is more compact than that of bulk HPW leading to a shrinkage of the lattice parameter by approximately 0.5 Angstrom but still accommodating a substantial fraction of the original water. Proton NMR supports the proposal that H3O+ is present and indicates that silica supported HPW is a much weaker Bronsted acid than unsupported HPW. NO is absorbed via the same mechanism by both unsupported and supported HPW to produce NOH+ species, although nu(N-O) bands in the IR are shifted to slightly different wave number in the supported samples. The rate of NO uptake is much lower for the supported catalysts, which may be explained by the much lower acid strength of these materials. Upon temperature programming to 723 K (50 K/min) more than 50% of the absorbed NO decomposes yielding nitrogen and nitrous oxide as the only reaction products.