On exposure of the solid 12-tungstophosphoric acid (H3PW12O40. nH(2)O (HPW)) to NO2 at 150 degrees C NO2 is sorbed, while HNO3 is produced. NO2 is found to penetrate into the bulk structure whose reaction occurs with the water hydrogen-bonded to the protons to produce HNO3. The NO2 retained on the catalyst is stoichiometrically equivalent to the total number of protons on and in the solid. This, together with infrared spectra, suggests that NO2 is associating (1/1) with the protons, producing a nitronium salt of the solid acid. Although insignificant quantities of NO are taken up by HPW, after exposure to NO2, approximately stoichiometric quantities of NO are sorbed and the infrared spectra show the presence of N2O3.