Sonolytic ozonation (US/O₃) is an effective way to degrade many pollutants in drinking water as the elevated mass transfer rate of ozone gas and the enhanced forming of hydroxyl radicals (^{radical dot}OH). This work investigated the formation of bromate (BrO₃⁻) from bromide (Br⁻) in sonolytic ozonation. At neutral pH, the bromate conversion rate ([BrO₃⁻]/[Br⁻]₀) was increased to 60% by ultrasound at continuous ozone flow (0–0.2 L min⁻¹), much higher than that without ultrasound or without bubbling. This indicates that the promoting effect of sonolysis on BrO₃⁻ formation is mainly due to the sonolytic decomposition of ozone and the enhancement of gas–liquid transfer. The [BrO₃⁻]/[Br⁻]₀ was increased with increasing pH. In addition, the reduction of HOBr/OBr⁻ with ultrasound demonstrates that bromate may be inhibited as the bromide was formed with the H₂O₂ generation under ultrasound. This suggests the competition between bromate and bromide during the US/O₃ led to the inhibition of bromate formation at high ozone flow. Therefore, our result reveals that the bromate formation under ultrasound is improved remarkably in US/O₃ in quick treatment with proper ozone flow (<0.2 L min⁻¹).