The sorption and conversion of NO and NO2 on the three heteropoly acids (metal-oxygen cluster compounds), 12-tungstophosphoric (HPW) 12-molybdophosphoric (HPMo), and 12-tungstosilicic (HSiW) acids are investigated and compared. Substantial quantities of NO2 are taken up by the tungsten-containing acids. In contrast, relatively small amounts of NO2 are sorbed by HPMo. In the absence of presorbed NO2 comparatively small quantities of NO are sorbed by any of the three solids. However, HPW and HSiW with presorbed NO2 sorb substantial quantities of NO. No evidence for reduction of either NO or NO2 is found. Temperature-programmed desorption, thermal gravimetry, infrared spectroscopy, and X-ray diffraction are employed to provide interpretive data. The results are interpreted as indicative of the formation of HNO3 from the reaction of NO2 and water as well as the substitution of the protons by NO2+ in establishing a charge balance mechanism.