The use of ultrasonic waves is one way of treating water laden with toxic and poorly biodegradable compounds. The aim of this study was to degrade an anionic dye belonging to the diazo family, reactive black 5 (RB5), by sonolysis. In order to optimize the sonochemical degradation of RB5, various parameters were studied, such as the initial pH of the solution, the initial concentration of the dye, the power of the ultrasonic radiation, and the effect of adding iron(II) ions and hydrogen peroxide. Sonochemical degradation of RB5 was monitored spectrophotometrically (UV/Vis). The results show that a yield of 73.8% is obtained after 180 minutes of treatment.