The combined effect of high power ultrasound and temperature on the activity of peroxidase type VI from horseradish suspended in water was studied. The tests were performed at 80 degrees C using power ultrasounds having frequencies of 20, 40, and 60 kHz. Accordingly, the actual ultrasonic powers varied in the range from 0 to 120 W. Combined treatments were carried out by using a laboratory scale plant operating in continuous and in batch mode. In continuous experiments, 46 mi of suspension was circulated in the plant having a sanitation chamber of 20 mi. In the other case, two sets of experiments were carried out by using a sanitation chamber of 100 mi treating 40 or 80 mi of suspensions, respectively. It was found that the decimal reduction time of peroxidase at 80 degrees C, D-80, reduces from 65 to 10 min ca. when ultrasounds are applied. In particular, D-80 varies with ultrasonic power, sonotrode geometry and volume of suspensions submitted to the treatment. It has been demonstrated that the influence of these variables on the decimal reduction time can be grouped by considering the ultrasound power density, i.e. the ultrasound power per unit area of tip of the probe and unit volume of suspension.