A method for controlling a selective catalytic reduction system (SCR) exploiting a set of predetermined trigger events and a set point for the NOx concentration, and an arrangement for SCR. At the occurrence of a trigger event, NOx concentration measurement downstream from the catalyst elements is started and the difference between the measured concentration and the set point is determined. If the difference is negative, the dosing of the reducing agent is decreased. If the difference is positive, the dosing of the reducing agent is increased and after system stabilization a new measurement is started. If the measured value is below the previous measured value, the same dosing is maintained. 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.