The influence of SO2 in the reduction of NOx by different potassium-containing coal pellets with catalyst loading between 7.9 and 16.8% was studied. For comparative purpose, a catalyst-free sample was also checked. At several temperatures between 350 and 550 degreesC, 2-h isothermal reactions were carried out under NOx/O-2/N-2 and NOx/SO2/O-2/N-2 gas mixtures, and the results revealed that SO2 inhibits drastically the potassium-catalyzed NOx-carbon reaction. The amounts of SO2 retained on samples indicate that there is a direct relationship between these quantities and the potassium loading on samples. X-ray fluorescence, Fourier transform infrared-in situ, and X-ray diffraction techniques were used in order to prove the presence of sulfur on samples after the reactions with NOx/SO2/O-2/N-2 and to elucidate the nature of the sulfur species formed. The cause of the loss in activity seems to be related to the SO2 irreversible chemisorption on the catalyst, originating the formation of potassium-sulfur compounds.