An unusual behavior was observed during experiments conducted at 371 and 465 degrees C with a commercial NOX storage/reduction catalyst: the outlet NOX concentration during the trapping phase reached a cycle minimum after regeneration, increased for 20-30 s, as expected, but then surprisingly decreased, indicating the catalyst had become more efficient in trapping the entering NOX. The results reported in this letter indicate that this phenomenon was caused by a superposition of two factors: a substantial exotherm associated with the regeneration event from reductant reacting with surface oxygen species and a negative impact of an evolving temperature profile on the dynamic NOX storage capacity.