We report that the regeneration of a Pt/BaO/Al2O3 NOx trap in H-2 occurs through the formation of NH3 as an intermediate. When NH3 is used instead of H-2, the reduction process is equivalent and equally effective to using H-2, thus confirming our hypothesis. The regeneration is localized, occurring at the plug flow front and is limited by mass transfer of the gas phase reductant to the catalyst. The process is consistent with the release of NOx from the trapping material, followed by its reduction over Pt. The high selectivity of Pt/BaO/Al2O3 in forming mostly N-2 during regeneration is achieved by the release of NOx only in the presence of H-2 which guarantees the formation of N-2 and NH3 and only small amounts of N2O. An oxygen source on the catalyst support, usually the stored NOx, is necessary for the oxidation of NH3 to N-2. (c) 2006 Elsevier Inc. All rights reserved.