Process for decreasing the content of nitrogen oxides in flue gases

A process for reductively decreasing the content of nitrogen oxide in flue gases that contain sulfur oxides, the process comprising contacting the flue gases with a catalyst and with NH.sub.3, which is employed as a reductant, said catalyst comprising at least one metal selected from the group consisting of titanium, zirconium, vanadium, tungsten, molybdenum and cerium in the form of one or more of their oxides and a silicate with a layer structure comprising acid-activated, but not yet X-ray amorphous kaolin whose crystalline layer structure is partly retained, the acid activation increasing the BET surface area at least 15% in terms of the BET surface area of the kaolin before acid activation, and the atomic ratio of the silicon in the acid-activated kaolin to the metal in the oxide being between 0.2 to 50, wherein the process is conducted at a temperature of 350.degree. C. to 600.degree. C., wherein the metal in the catalyst is present in an amount of 0.1 to 80% by weight, with the acid-activated kaolin accounting for the rest of the catalyst weight.