Thermally stable, solid alumina-based compositions useful for the trapping of nitrogen oxides that are contained in gaseous fluids, notably in vehicular exhaust gases, are formed of alumina, cerium and a divalent metal M selected from among barium, strontium or an association of these two elements and which include at least 10% of the weight of element M expressed by weight of oxide in contrast to the total weight of the composition and also wherein the aluminate compound of the metal element M, i.e., MAl204, is absent and not detectable by X-ray diffraction of the composition, after calcination thereof with air at 700° C. for two hours.