Lead is one of the poisonous elements for de-NOx catalysts especially in the case of municipal waste incineration plants. Samples of a commercial selective catalytic reduction catalyst were impregnated with different amounts of Pb(NO3)(2), and their catalytic activities were studied. The XPS and SEM analysis showed higher lead concentration on the outer surface of the monolith than on the inside of the wall for all samples. Pb covers the surface of inactive TiO2 sites, likewise active V2O5/WO3 sites, in a thin, noncrystalline layer. Both activity and NH3 chemisorbed decreased in the same manner with increasing poison coverage. This suggests that the area available for reaction was the same or at least proportional to that for the adsorption of NH3 and deactivation of the catalyst may be due to competitive chemisorption of the poison on the acid sites instead of by pore blocking.