Process for catalytic treatment of exhaust gases

A process for removing nitrogen oxides (NO.sub.x) from an exhaust gas largely avoiding formation of inactivating deposits on the catalyst comprising the steps of forming a mixture of ammonia and the exhaust gas and contacting the resulting mixture with a reducing catalyst at a temperature from about 150.degree. to 250.degree. C. wherein the time between the formation of the mixture and the occurrence of contact between the mixture and the catalyst is at most 1 second.