A process for the selective catalytic reduction of nitrogen oxides with ammonia in a lean-mix exhaust gas from a combustion process operated with a first, lean-mix air/fuel mixture, wherein the ammonia required for selective reduction is obtained from a second, rich-mix air/fuel mixture which contains nitrogen monoxide by reduction of the nitrogen monoxide in a NH.sub.3 synthesis step to give ammonia with the formation of a product gas stream, and where the nitrogen monoxide-containing second air/fuel mixture is supplied to the NH.sub.3 synthesis step for reduction of the nitrogen monoxide to ammonia over a three-way converter catalyst and the ammonia formed is separated from the product gas stream and is stored in a storage medium for use as and when required for selective catalytic reduction.