A new method for on-line process identification and PI controller tuning is proposed. In the method proposed a standard PI control loop is tuned to become unstable through aggressive controller settings, and the obtained oscillations are used for process identification and subsequent controller tuning. The control loop can be tuned to be unstable by mistake or on purpose. In the case where the control loop turns unstable by mistake, due to, e.g., changes in the operating conditions, this new method is useful as it provides a possibility for instant controller retuning of oscillating control loops. The control loop can also be tuned to be unstable on purpose. In that case the method has proved to be particularly suitable for process startup, because it provides a more rapid startup procedure than traditional methods do. With the proposed method, amplitude-and-phase-margin PI controller tuning is utilized. The method is tested on an experimental laboratory process with two inputs and two outputs.