The sonochemical oxidation of As(III) in the presence of peroxydisulfate ion (PDS) has been investigated. Sulfate anion radicals and OH radicals produced during acoustic cavitation readily oxidized As(III) to As(V) in an aqueous environment. The rate of oxidation of As(III) was remarkably high (∼10 times) with respect to the concentration of PDS. The As(III) oxidation was found to be independent of the initial pH of the solution in the range 3–8. It was relatively low at pH above 8, however, this could be circumvented by increasing the concentration of PDS. The presence of oxygen in solution played a significant role in the rate of oxidation of As(III). Around 40% oxidation of As(III) was observed in the absence of oxygen compared to 80% oxidation in the presence of dissolved oxygen (10 mg/L) over a sonication time of 5 min. The addition of humic acid (HA) retarded the oxidation rate of As(III), but the effect could be offset by using larger amounts of PDS. The effects of ultrasound intensity, and frequency on the rate of the oxidation of As(III) were also studied. The rate of the oxidation of As(III) was not significantly dependent on the acoustic power applied, for the concentrations of As(III) used in this study. At an ultrasound frequency of 211 kHz, the rate of oxidation of As(III) was lower than that observed at 20 kHz. It is concluded that the sonochemical treatment of As(III) solutions in the presence of PDS is a simple and viable technique for the oxidation of As(III) to As(V).