A method for controlling a selective catalytic reduction system (SCR) exploiting a set of predetermined trigger events (101) and a set point for the NOx concentration(102). At the occurrence of a trigger event(104), NOx concentration measurement downstream from the catalyst elements (2) is started(105)and the difference between the measured concentration and the set point is determined(106). If the difference is negative, the dosing of the reducing agent is decreased (107) and a new trigger event is waited for (103). If the difference is positive, the dosing of the reducing agent is increased(108). After system stabilization(109), anew measurement is started (110) and the concentration is compared to the result of the previous measurement(111). If the measured value is below the previous measured value, the same dosing is maintained(112). If the measured value is above the previous measured value, the dosing is decreased with an amount greater than the increase after the previous measurement(113).The invention also concerns an arrangement for SCR.