The influence of reductant supply timing on the performance of a Ba/Pt-Al2O3 model storage-reduction catalyst was analysed. This study was developed in terms of NOx and CO conversion, CO availability for NO ad-species reduction and temperature profile analysis. A relevant CO-O-2 mixing effect was noticed during lean-to-rich and rich-to-lean transitions. On the basis of constant overall amount of reductant, the intensity of this effect was dependent on the number of transitions per time unit. Then, an excessive cycle length reduction led to operation states with CO defect, that subsequently controlled NOx conversion and temperature profiles over the catalyst. This mixing effect was related to the deviation from ideal plug flow condition. In this real-flow context, and due to the inherently transient NSR operation, the need for a quantitative description of those departures from idealised flow was evidenced. A first approach, just for the case of a discrete lean-to-rich transition, was presented in this work. (C) 2009 Elsevier B.V. All rights reserved