A four-step mechanism is proposed to describe the reduction of NO, by potassium containing coal char pellets under O-2-rich atmospheres. The four-step mechanism includes the chemisorption of both O-2 (step 1) and NOx (step 3) on the so-called free sites (C-f) of carbon, which generates surface oxygen complexes (denoted by (CO)(#)). The step 2 considers the direct decomposition of (CO)(#) to yield CO2 and the step 4 the reaction between NOx and (CO)(#) to yield CO2 and C-f. NOx reduction isothermal reactions between 350 and 450 degrees C have been carried out with potassium containing coal char pellets (16.8% w/w of catalyst) under 0.2%NOx/5%O-2/N-2 and 0.2%NOx/2%H2O/5%O-2/N-2. The NOx reduction and sample conversion experimental profiles have been successfully simulated by the system of algebraic and differential equations deduced from the four-step mechanism, which indicates that the mechanism seems to be feasible. Experimental results pointed out that the selectivity of pellets towards NOx reduction against O-2 combustion decreases with temperature. This is in agreement with the elemental step rate constants (k(step) (number=) (1, 2, 3, or 4)) predicted by the model, that is, k(1) and k(2) increased with temperature in a major extent than k(3) and k(4), which are the steps in which NOx, are involved. The selectivity also decreases when H2O is present in the reactive mixture. This is due to the destabilisation of (CO)(#) in the presence of H2O, thus creating Cf through step 2 that react with O-2 (step 1) in a major extent than with NOx. (step 3), as it is also deduced from the elemental step rate constants predicted by the model. (c) 2005 Elsevier B.V. All rights reserved.