The activity of potassium-containing char briquettes, for NOx reduction, has been investigated at 350 degreesC, using a mixture of 0.2%NO/5%/O-2/N-2. The samples have been prepared from a Spanish bituminous coal and a coal tar-formaldehyde resin (resole) as binder. The influence of a number of variables relating to the preparation process, such potassium loading and pyrolysis temperature, on their reduction capacity has been analyzed. For comparative purposes, a set of binder-free samples with different potassium contents has been prepared. Potassium contents highly affect the selectivity of the briquettes. Samples prepared with high potassium loading exhibit satisfactory selectivity values with very low burnoff. Lifetime tests, until complete consumption of the sample, have been performed with the samples exhibiting high selectivity values: The most important finding is that high selectivity values are maintained during the whole life of the briquette. The binder affects the reaction kinetic and the lifetime of the sample. A binder-free sample presents higher activity with regard to binder samples.