Various high-frequency (500 kHz) devices were investigated to determine the optimal geometry of sonoreactors used for wastewater treatment. Several complementary measurements (distribution of residence time, jacketed thermocouple, chemiluminescence of luminol, direct piezoelectric effect, calorimetry, sodium pentachlorophenate degradation) were used to characterize the acoustic field. These measurements evaluate either local or global physical or chemical ultrasonic activity. It was shown that in some specific reactors, standing waves can settle in the liquid, causing high energy spots to appear at pressure antinodes. It was found that these conditions provide greater sonochemical efficiency.