A method is described for reducing nitrogen oxides in the exhaust gas flow of combustion engines, especially a motor vehicle internal combustion engine, wherein an air/fuel mixture in the combustion chamber of the combustion engine is combusted in the presence of a fuel additive and, in the process, the fuel additive decomposes and the exhaust gas flow is guided through at least one catalyst arranged downstream of the combustion chamber. This method is characterized in that a fuel additive is used, which decomposes in the combustion chamber releasing ammonia and the exhaust gas flow containing ammonia is guided through at least one SCR catalyst arranged downstream of the combustion chamber to reduce the concentration of the nitrogen oxides contained therein. The method according to the invention is highly effective in reducing the nitrogen oxide content in the exhaust gas flow of a combustion engine without an exhaust gas aftertreatment system having to be expensively formed and the installation space having to be increased.