During the preparation of alumina as a catalyst support from aluminium nitrates by precipitation with a NH4OH base, NO2. radicals have been formed in the catalyst after calcination under air in the solid at different temperatures. These radicals remained stable until a calcination temperature of 800 degrees C. When the calcined catalyst was degassed under vacuum above 300 degrees C, the NO2. was reduced to give NO. and O-species which were both tightly trapped in the solid. These latter species remained stable until vacuum treatment at 800 degrees C.