Polyphenoloxidase (PPO) and peroxidase (POD) are enzymes commonly found in fruits and vegetables. Food processing mainly aims the inactivation of those enzymes since they are often associated with browning, off-flavors and reduction of nutritional value. Ultrasound technology has emerged as an alternative to conventional thermal processing for enzymatic inactivation. The aim of this work was to evaluate the influence of the ultrasound processing on the residual activity of POD and PPO. For this purpose, a fruit juice model solution (10 °Brix and pH 3.8) were prepared using saccharose, citric acid and horseradish POD (4 U.mL-1) or mushroom PPO (16 U.mL−1). The solutions were processed for up to 3 h using an ultrasonic bath with a fixed frequency of 25 kHz and an acoustic density of 26 W.L−1. Samples were placed in glass beakers positioned in a specific location inside the ultrasonic bath containing distilled water. A heat exchanger with circulation of cold water was used to control the sample temperature (23 ± 1 °C). The actual volumetric power delivered to the samples (20 W.L−1) was determined following the calorimetric method. POD and PPO activities were determined spectrophotometrically at 420 nm using pyrogallol and 4-methylcatechol as substrates, respectively. A significant variation in PPO activity was observed as function of processing time. After 0.5 h of processing, the PPO activity increased ~24%. Then, the PPO activity decreased over the processing time, reaching ~77% of the initial activity after 1.5 h and ~48% after 3 h. The observed activation and inactivation were a result of conformational changes and molecular unfolding. On the other hand, sonication had no considerable effect on POD activity. The obtained results indicate that ultrasound can modify the enzymatic activity, being able to either activate or inactivate, depending on enzyme and processing time.