In this work the activity of potassium-containing coal-pellets against NOx at 450degreesC was studied. The sample chosen presents high potassium content, 16.8% in wt, and was prepared from a humic acid+KOH mixture and a Spanish high volatile A bituminous coal. The effect of the NOx partial pressure, combined with that of the influence of the residence time as variables for the NOx reduction process were investigated in order to look for reaction conditions where high NOx conversions be reached and maintained during a long time. 100% of NOx conversion was reached and kept during a long time, employing a sample mass of 2 grams (residence time 1.8 s), regardless of the NOx partial pressure tested. A first reaction order, with respect to NOx partial pressure, was found from the fit of all the data, in agreement with the data reported in the literature. The rate constant estimated at the temperture of study was of 1.15 s(-1).