A series of Mn-exchanged zeolite catalysts was tested for its DeNO(x) activity in ammonia selective catalytic reduction (NH3-SCR) reactions. Activity was found to depend strongly on metal loading such that at low Mn content, NOx conversion is limited to 50% at 700 K. However, above a mass percentage of Mn between 1.6 and 1.9%, near complete NOx conversion is achieved at 525 K. Ammonia - temperature programmed desorption (NH3-TPD) was used in an attempt to explain the findings of these experiments. Specifically, we have used NH3-TPD to determine the population and strength of the acid sites present in the catalysts as a function of Mn content. Acid sites with adsorption energies ranging from 45 to 68.7 kJ/mole have been detected. At temperatures where DeNO(x) activity differs significantly among the catalysts, corresponding differences in their acid site distributions, and hence surface NH3 populations, are observed. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.