The desulfation mechanisms of pre-sulfated Pt-BaO/gamma-Al2O3 lean NOx trap catalysts were investigated under isothermal conditions (600 degrees C) using H-2 as the reductant. Sulfates were found to be reduced first with H-2 to produce SO2, followed by a reaction between SO2 and H-2 to produce H2S. Gas analysis during the rich pulse reveals that the sulfur removal efficiency is initially proportional to the H-2 concentration. At constant H-2 concentration the overall desulfation efficiency decreases in the order of H-2/CO2/H2O > H-2/CO2 > H-2/H2O > H-2, as confirmed by XPS analysis of residual sulfur in the desulfated samples. H2O limits the evolution of SO2 at an early stage of the rich pulse and enhances the production of H2S in later stages of reduction. CO2 is involved in both the formation of COS and the production of H2O (via the reverse water-gas shift reaction), therefore, resulting in an increased overall efficiency. (C) 2011 Elsevier B.V. All rights reserved.