In this study, we compared the NOₓ storage reduction characteristics of four systems: BaO/Al₂O₃ alone, a sequential system with Pt/SiO₂ ahead of the BaO/Al₂O₃, a combined system with the Pt/SiO₂ and BaO/Al₂O₃ physically mixed, and BaO/Al₂O₃ with Pt deposited on it. We also investigated the isotopic exchange between ¹⁵NO and stored NOₓ under storage conditions for the latter two systems. The rate of exchange was more than five times as fast for Pt/BaO/Al₂O₃ than for the combined Pt/SiO₂ + BaO/Al₂O₃ system, demonstrating spillover of NOₓ species between Pt and BaO in close proximity. Movement of NOₓ in this way can also explain why storage from NO/O₂ reached completion much faster for Pt/BaO/Al₂O₃ than for the sequential or combined systems, where the influence of Pt was confined to the oxidation of NO to NO₂ with storage by disproportionation to nitrate and NO alone. The initial product of the decomposition of stored NOₓ in He was NO₂ with the sequential system and NO with the combined system, where Pt in the same bed decomposed NO₂. However, NO₂ in excess of the NO₂/NO equilibrium ratio was seen during release of stored NOₓ from Pt/BaO/Al₂O₃, reflecting transfer back to the metal, which was deactivated for NO₂ decomposition by high oxygen coverage. A large fraction of the NOₓ stored on Pt/BaO/Al₂O₃ was immediately converted to N₂ alone when exposed to H₂, with formation of NH₃ evident only after H₂ breakthrough. This is consistent with the reduction of stored NOₓ species as they transfer back to Pt particles. Reduction in the combined system requires considerably higher temperature and forms NH₃ as the major product. This is consistent with the decomposition of stored NOₓ to gaseous NOₓ and subsequent reduction by a catalytic reaction on Pt particles located remotely from BaO. In the sequential system, reduction was largely confined to the conversion of NO₂ to NO. Measurement of the O:N ratio in the products formed by temperature-programmed decomposition showed that NO was stored on BaO/Al₂O₃ in the presence of O₂ as a species with the formula NO₂. Similar measurements with NO₂, which is stored in much larger quantities, were consistent with conversion to nitrate, in agreement with existing knowledge.