The presence of NO during the regeneration period of a Pt-Ba/Al2O3 Lean NO Trap (LNT) catalyst modifies significantly the evolution of products formed from the reduction of stored nitrates, particularly nitrogen and ammonia. The use of isotope labelling techniques, feeding (NO)-N-14 during the storage period and (NO)-N-15 during regeneration allows us to propose three different routes for nitrogen formation based on the different masses detected during regeneration, i.e. N-14(2) (m/e = 28), (NN)-N-14-N-15 (m/e = 29) and N-15(2) (m/e = 30). It is proposed that the formation of nitrogen via Route 1 involves the reaction between hydrogen and (NO5)-N-14 released from the storage component to form (NH3)-N-14 mainly. Then, ammonia further reacts with (NOx)-N-14 located downstream to form N-14(2). In Route 2, it is postulated that the incoming (NO)-N-15 reacts with hydrogen to form (NH3)-N-15 in the reactor zone where the trap has been already regenerated. This isotopically labelled ammonia travels through the catalyst bed until it reaches the regeneration front where it participates in the reduction of stored nitrates ((NOx)-N-14) to form (NN)-N-14-N-15. The formation of N-15(2) via Route 3 is believed to occur by the reaction between incoming (NO)-N-15 and H-2. The modification of the hydrogen concentration fed during regeneration affects the relative importance of H-2 or (NH3)-N-15 as reductants and thus the production of N-14(2) via Route 1 and (NiN)-Ni-14-N-15 via Route 2. (C) 2011 Elsevier Inc. All rights reserved.