A method and a device for determining a recirculation during a dialysis on the basis of the response of the dialysis system to an alteration of a system-related operating value are described. Recirculation may be determined by establishing, at the side of the dialysis liquid, a reference parameter that represents the concentration of uremic toxins from the extracorporeal circulation, and calculating a target parameter for a system-related target operational value from the reference parameter, presetting the system-related target operational value and establishing an actual parameter related to the system-related target operational value, determining an actual comparative value from the actual parameter and the reference parameter related to the system-related target operational value and comparing the actual comparative value with a system-specific target comparative value related to the system-related target operational value, and converting the comparison result into a recirculation degree.
