A method and an emission control system are for catalytically reducing nitrogen oxides in the exhaust gas of a combustion system, especially a diesel motor. An oxidation catalyst is arranged upstream in relation to the reduction catalyst and in the exhaust gas channel. A reaction agent for decomposing nitrogen oxides according to the method of the selective catalytic reduction is added according to the temperature and upstream or downstream in relation to the oxidation catalyst and upstream in relation to the reduction catalyst. The selectivity of the oxidation catalyst is used for decomposing nitrogen oxides to form molecular nitrogen.