A process for reducing nitrogen oxides to nitrogen from exhaust gases is disclosed, which involves bringing an exhaust gas containing nitrogen oxides into contact with at least one catalyst from proton-type zeolites, zeolites of an alkali metal exchanged form and acidic metal oxides, by reaction with a hydrocarbon or an oxygen-containing organic compound in an oxidizing atmosphere containing excess oxygen. A process which further involves bringing the exhaust gas into contact with an oxidizing catalyst subsequent to the above process.