Abstract: A self tuning control system and a method for tuning such a system. The system has a controller formed by a PI controller and a proportional control unit with a mixing point therebetween to which the absolute reference valve is connected. The system has another mixing point upstream of the controller to which the reference variable and the absolute reference valve are connected. The self tuning method involves two cycles. In the first cycle the control loop of the controller is closed only through the proportional control unit and the proportional component of the proportional control unit is determined. In the second cycle the control loop is closed through both the PI controller and the proportional control unit and the integral action time and proportional action coefficient of the PI controller are determined.