Abstract: Nitrogen oxides are effectively removed from a gaseous mixture comprising the same and oxygen by adding ammonia thereto and contacting with a suitable catalyst for the nitrogen oxide-ammonia reaction. While the presence of sulfur dioxide is not essential to such removal, best results are achieved when sulfur dioxide is also present in the gaseous mixture. When sulfur dioxide is present, and particularly when the same is present in a concentration in excess of that required for most effective nitrogen oxide conversion, the excess sulfur dioxide may be separated prior to the nitrogen oxide conversion, simultaneously therewith or subsequent thereto.