Many azo dyes are consumed in the textile and dyeing industry, which makes the wastewater recalcitrant and toxic to the aquatic environment. Dye degradation by the combination of hydrodynamic cavitation and ozone (HC + O 3) has caused extensive interest. The degradation mechanism of the hybrid system needs further investigation. This study investigated the degradation of acid red 73 (AR73) by HC + O 3. Meanwhile, the degradation pathways and mechanisms were present. The optimal operation parameters were: inlet pressure of 0.15 MPa, O 3 dosage of 45 mg/min, initial dye concentration of 10 mg/L, and initial pH at 7.5. As a result, the decolorization rate, removal of UV 254 and NH 3 –N were 100%, 71.28%, and 87.36% in 30 min, respectively. Humic acid and most of the co-existing anions (H C O 3 − , S O 4 2 − , C l − , P O 4 3 − , N O 3 −) played a positive role in the degradation of AR73, while N O 2 − restrained. The reactive species of singlet oxygen (1O 2), hydroxyl radicals (·OH) and super oxygen radicals (· O 2 −) showed synergism in the hybrid system, and the decolorization was attributed to the fracture of azo bonds by 1O 2. Meanwhile, aromatic amines were generated and further degraded into small molecule compounds. The research certificated that the HC + O 3 can be an effective technology for azo dye degradation. [Display omitted] • The degradation of AR73 was 100% in 10 min in the hybrid system of HC + O3. • HCO3 -, SO4 2-, Cl-, PO4 3- and NO3 - promoted AR73 degradation, while NO2 - restrained. • 1O2, ·OH and ·O2 - worked together to degrade dyes and 1O2 played a key role. • The decolorization of AR73 was mainly due to the cleavage of azo bonds. • Aromatic amines were formed in the degradation and then effectively degraded. [ABSTRACT FROM AUTHOR]