Differences in the NO x storage-reduction (NSR) behavior of Pt/Ba/CeO 2 and Pt/Ba/Al 2 O 3 have been identified and traced to their different chemical and structural properties. The results show that Pt/Ba/CeO 2 exhibits inferior NO x storage and, particularly, reduction (regeneration) activity compared to the Al 2 O 3 supported catalyst. The incomplete reduction of the stored NO x -species in Pt/Ba/CeO 2 seems to be caused by a faster and more profound reoxidation of Pt particles during the lean period as evidenced by in situ X-ray absorption spectroscopy. Interestingly, the reduction activity could be significantly improved by a pre-reduction step at mild conditions. Exposure of the Pt/Ba/CeO 2 catalyst to reducing H 2 atmosphere in the temperature range 300–500°C lead to a moderate increase of Pt particle size which beneficially influenced the regeneration activity. In contrast, pre-reduction at temperatures above 500°C was unfavorable and resulted in a severe decrease of the regeneration activity, probably due to migration of the partially reduced CeO 2 onto the surface of Pt particles.