Lean NO trap catalysts composed of Pt/BaO-Al2O3 represent a viable approach to meet the stringent NO emission standards being imposed on diesel engines. A series of Pt/BaO-Al2O3 catalysts with three different sulfation levels (non-sulfated, partially-sulfated, and fully-sulfated) were prepared. Pt accessibility after successive reductions in H2 at elevated temperatures was measured by an irreversible volumetric H2 chemisorption method, and the morphology of the sample after reduction at 800°C was characterized. Pt accessibility and morphology changes had a complex dependence on the levels of sulfation. The H/Pt ratio decreased uniformly as a function of reduction temperature on the non-sulfated sample even though Pt particle sizes showed negligible changes. Compared to the non-sulfated sample, the partially-sulfated behaved quite differently, instead displaying an optimum H/Pt value at 500°C and slight increase in Pt particle sizes after thermal aging at 800°C. This is an abstract of a paper presented at the AIChE Annual Meeting (San Francisco, CA 11/12-17/2006).