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The aim of this work was to study the formation of ammonia during the regeneration step of a commercial lean NO xTrap catalyst in real conditions. Experiments were carried out on an engine test bench equipped with 2.2 L common rail diesel engine. The after-treatment system was composed of a commercial NSR monolith catalyst with a volume of 2.2 L. This material was composed of platinum, palladium and rhodium, as active components, with barium oxide and alumina as storage component and support, respectively. Catalytic measurements were carried out at 280 °C. Particular attention has been paid to formation of ammonia during the purge using hydrogen and CO as reducing agents. The time dependency of the extent of ammonia formation and the influence of hydrogen concentration were carefully examined. It was observed that the temperature rose in the first step of the regeneration process, after the switch in rich conditions, due to the occurrence of exothermic oxidation (H2/O2, CO/O2) accompanied with the releasing of available vacant noble metal sites which may provide a route for H2 and NO dissociation and/or subsequent surface reactions leading to the ultimate formation of ammonia. It was found that ammonia formation strongly depends on the purge duration and on the concentration of the reducing agent available during the purge. [ABSTRACT FROM AUTHOR]