In this work, the degradation of Basic Red 29 (BR29), an azo dye, was investigated using a high-frequency ultrasonic reactor (300 kHz). The influence of several parameters such as initial substrate concentration (5--200 mg L$^{-1})$, pH (3-10.1), ultrasonic power (20-80 W), and nature of the dissolved gas (Ar, air, and N$_{2})$ on the degradation of BR29 was assessed. Additionally, the impact of natural matrices (seawater and natural and river waters) on the degradation rate of BR29 was clarified. Degradation experiments with radical scavengers, tert-butyl alcohol and glucose, showed that BR29 degraded mainly through HO$^{\bullet }$ radical attack at the gas/liquid interface of the cavitation bubbles. The degradation of the dye was strongly sensitive to the operational conditions. The natural matrices enhanced the degradation of the dye. Ultrasonic mineralization and oxidation of the dye solution was significantly enhanced by addition of a low quantity of Fe$^{2+}$ (15 mg L$^{-1})$.