The invention relates to a device and method for determining parameters of a welding system. According to the invention, a welding area (18, 21, 22) is subjected to the action of ultrasonic waves, preferably to the action of shear waves, by using an ultrasound source (14). During a first welding process (n1), a signal processing (30) determines a first ultrasonic permeability (Dn1) from a received ultrasonic signal (UE). During at least one subsequent welding process (n2), a second ultrasonic permeability (Dn2) is determined from an ultrasonic signal (UE) that is received during a renewed exposure of the welding area (18, 21, 22) to ultrasonic waves. A display (32) and/or a diagnostic function and/or a correction of control quantities of the welding system is/are carried out as a function of the at least two ultrasonic permeabilities (Dn1, Dn2).