The invention relates to a method for controlling injection of a reductant into a NOx containing exhaust gas stream from a combustion engine, where combustion of a known fuel takes place and where the HOx is reduced by selective catalytic reduction, SCR, in the presence of a SCR catalyst, comprising establishing signals related to exhaust gas flow, HOx concentration in the exhaust gas stream upstream of the catalyst, exhaust gas temperature upstream of the SCR catalyst, exhaust gas temperature downstream of the SCR catalyst and to input data for catalyst characteristics. From the signals an adjusted amount of the reductant is calculated by using a calculated amount of reductant and a value d(E*T)/dt, where E is the exhaust gas flow, T is the exhaust gas temperature upstream of the catalyst and t is time.