The effect of axial length on the NO, reduction performance of two different commercial Lean NOx Trap (LNT) monolithic catalysts was experimentally investigated in a bench flow reactor. The washcoat composition of one of the catalysts consisted of Pt and K on gamma-Al2O3; whereas the other catalyst contained a complex mixture of Pt, Pd, Rh, Ba, Ce, Zr, Mg, Al and others. The NOx removal characteristics of cylindrical monolith segments of constant diameter (2.22 cm) and axial lengths of 2.54, 5.08 and 7.62 cm were evaluated using a simulated lean engine exhaust containing water and carbon dioxide at a constant space velocity of 30,000 h(-1). No significant effects of length were observed when the catalysts were fully reduced with hydrogen between NOx capture phases. However when the catalysts were only partially regenerated NO, reduction efficiency increased with monolith length. Intra-catalyst H-2 measurements at different axial locations indicated that at least some of the efficiency loss during partial regeneration occurred when back-mixed H-2 was directly oxidized and became unavailable for nitrate reduction. (C) 2009 Elsevier B.V. All rights reserved.