The oxidation of NH3 over three different types of partially sulphated limestone is investigated experimentally and theoretically. The experiments were made in a laboratory fixed bed reactor at atmospheric pressure at 850degreesC to simulate conditions of fluidized bed combustion. The catalytic activity of calcined limestone decreases strongly when it is sulphated. The decrease in the rate of NH3 oxidation is mainly caused by the conversion of CaO into less active CaSO4 and by the formation of an impervious layer of CaSO4 around the CaO core. A model based on the grain model has been developed to describe the decrease of activity for NH3 oxidation with increasing conversion of CaO to CaSO4. The changes in structure and in composition of the limestone particle are incorporated in the model. The simulation results are in good agreement with the experimental data. Besides the decrease in activity, the selectivity for the formation of NO decreases during sulphation of the limestone. In experiments, it was shown that the decrease in selectivity is independent of the NO concentration and therefore is not the result of NO reduction by NH3 as proposed in the literature. Further, it was shown that while the formation of NO decreases with increasing CaO conversion the formation of N-2 is almost not influenced. From these observations and from simulation results it was concluded that the NH3 oxidation to N-2 over CaSO4 is the major cause of the decrease in selectivity. To our knowledge, this is the first validated explanation for the decrease in selectivity for NO during sulphation of calcined limestone. (C) 2004 Elsevier Ltd. All rights reserved.