This paper aims to evaluate the de-NOx performance of a combined system of LNT + SCR catalysts according to the durability of hydrothermal aging and sulfur poisoning. NOx conversion, NH3 production, and N2O emission from the LNT and the combined system of LNT and SCR were evaluated according to the conditions of hydrothermal aging (750-900 degrees C) and sulfur poisoning (1.0-1.48 g/L). Regarding the NOx-TPD characteristics, it was found that the amount of NOx desorption decreased due to hydrothermal aging and sulfur poisoning. The NOx conversion of the combined system of LNT and SCR (aged at 750 degrees C and 900 degrees C) was 10-30% higher than that of the LNT system. During this time, the maximum (NH3)(produced)/(H-2)(inlet) ratio of the LNT catalyst was 0.81%, which the produced NH3 reacted with NOx as a reductant at the downstream SCR catalyst. These characteristics are some merits that compensate the reduction in catalyst activity due to hydrothermal aging. The NOx conversion of the 1.48 g/L sulfur-poisoned LNT + SCR system was 20% lower after poisoning than before poisoning, and the NOx conversion of the LNT + SCR system did not recover due to excessive sulfur poisoning even after desulfation. The N2O emission of the hydrothermally aged catalyst was small because the NOx quantity stored on the LNT was reduced. The combined system of LNT and SCR catalysts had the merit of compensating the de-NOx performance, and reducing the NH3 slip and the N2O emission, even though the catalysts were aged hydrothermally. (C) 2010 Elsevier B. V. All rights reserved.