Simulations and real decomposition experiments with hydrogen peroxide induced by dissolved iron and solar illumination were performed in a solar pilot plant with compound parabolic collectors designed for photo-Fenton wastewater treatment. The structure of a gray-box linear model aimed at reproducing system dynamics with parameters dependent on operating conditions when operating around prescribed set-points is proposed. This simple model relates the changes in hydrogen peroxide concentration due to changes in hydrogen peroxide injection, dissolved iron, and solar illumination. Based on this model, control of the hydrogen peroxide concentration at set points between 200–900 mg/L±10 mg/L under dynamic conditions (simultaneous decomposition and addition of hydrogen peroxide by a dosage pump), was achieved. Different basic approaches for controller actuation upon the frequency of the dosage pump were tested, ranging from a simple PI controller to a PI controller plus antiwindup action and feedforward control. From these basic approaches, conclusions can be drawn about the process's behavior under closed-loop control