The invention relates to a method and a device for monitoring a flow of liquid for the presence of air bubbles, in particular the blood flowing in an extracorporeal blood circuit of an extracorporeal blood treatment device. The air bubbles are detected by an ultrasound measurement and the ultrasound signals received in a continuous sequence of time intervals are each compared to a predetermined reference level. When the amplitude of the ultrasound signal, or a parameter correlating to the amplitude of the ultrasound signal, is less than the predetermined reference level, the presence of a defined volume of air is determined. When the number of times the ultrasound signal is less that the predetermined reference value if is greater than the set limit value, it is determined that a critical volume of air is present in the patient's blood and the dialysis is stopped.
