A treatment system for providing treatment of an exhaust stream comprising nitrogen oxides NOx, in which nitrogen monoxide NO and nitrogen dioxide NO2 are comprised. When the exhaust stream passes through the treatment system, oxidation occurs of compounds comprising nitrogen, carbon and/or hydrogen. An amount of nitrogen oxides NOx reaching a reduction catalyst device downstream of the oxidising component in the exhaust treatment system is reduced. A ratio (NO2/NOx)det between an amount of nitrogen dioxide NO2 reaching a reduction catalyst device and the amount of nitrogen oxides NOx reaching the reduction catalyst device is determined. An active control of at least one parameter related to the combustion engine is carried out, based on the determined ratio, so that the amount of nitrogen oxides NOx reaching the reduction catalyst device is increased, if the determined ratio (NO2/NOx)det exceeds an upper threshold value (NO2/NOx)threshold_high.