The interaction of NO, NO2, or NO/O-2 mixtures with Pt/Al2O3, Ba/Al2O3, and Pt-Ba/Al2O3 catalysts has been investigated by IR spectroscopy and temperature-programmed desorption. Upon NO interaction, small amounts of nitrites, nitrates, and hyponitrite species were formed on the Ba-containing samples. The NO, storage capacity of the catalysts was highly enhanced upon adsorption of NO/O-2, mixtures and further upon NO2 admission. Upon adsorption of NO/O-2 on Pt/Al2O3 sample nitrites, nitrates and NO2delta+ species were mainly formed, showing a moderate thermal stability. Barium markedly increased the amount and stability of the stored NO, species, which were bidentate and monodentate nitrites and, in minor a-mounts, nitrates. Nitrites were removed below 750 K and/or transformed into ionic Ba nitrates, stable up to 800-900 K. Upon NO2 adsorption, huge amounts of nitrates, but no nitrites, were formed on all the samples. Also in this case, Ba increased the amount and stability of the stored NO, species, The nature and the amounts of the stored NO, species formed upon adsorption of NO, NO/O-2, or NO2 were similar on Ba/Al2O3 and Pt-Ba/Al2O3 catalysts, whereas Pt slightly decreased their thermal stability. Bulk), Ba nitrate was formed during the thermal desorption of NO2 (and to a less extent of NO/O-2), inducing an extensive decomposition of the Ba carbonate or oxycarbonate phase which was detected on the calcined samples.