The reduction of NO with acetaldehyde on alumina-supported Pd, Mo, and Pd-Mo catalysts was studied. Two Mo loadings were used (8 and 20%), and the Pd-8% Mo/Al2O3 catalyst was the most active for NO conversion. Temperature-programmed desorption and infrared measurements of adsorbed acetaldehyde indicated that acetate species are probably an important reaction intermediate. The presence of Pd in close contact with Mo favors formation of the acetate species, which could explain the higher activity for the Pd8Mo sample. The 20% Mo loading produced bulk MoO3, which covered part of the Pd sites, and thus the activity for this catalyst was lower. (c) 2006 Elsevier Inc. All rights reserved.