Ultrasonic irradiation in the presence of ozone is demonstrated to be effective for the rapid oxidation of oxalic acid, bioxalate, and oxalate (H2C2O4/HC2O4-/C2O42-) in aqueous solution to CO2 and H2O. The degradation rate of bioxalate exposed to "sonozone" (i.e., simultaneous ultrasonication and ozonolysis) was found to be 16-times faster than predicted by the linear addition of ozonolysis and ultrasonic irradiation rates. The hydroxyl radical ((OH)-O-center dot) is the only oxy-radical produced that can oxidize oxalate on a relevant time-scale. Thus, plausible (OH)-O-center dot production mechanisms are evaluated to explain the observed kinetic synergism of ultrasonication and ozonolysis toward bioxalate decomposition. (OH)-O-center dot production via decomposition of O-3 in the cavitating bubble vapor and via the reaction of O-3 and H2O2 are considered, but kinetic estimations and experimental evidence indicate neither to be a sufficient source of (OH)-O-center dot. A free-radical chain mechanism is proposed in which the HC2O4- + (OH)-O-center dot reaction functions as a primary propagation step, while the termination occurs through the O-3 + CO2 center dot- reaction via an O-atom transfer mechanism. Kinetic simulations confirm that ozone reacts efficiently with the superoxide (O-2) ion that is produced by the reaction of O-2 and CO2 center dot-to form (OH)-O-center dot radical, and that the reaction of O-3 + CO2 must be chain terminating. Oxalate is also readily oxidized by "peroxone" treatment (i.e., H2O2 and O-3). However, the addition of H2O2 during the course of the sonolytic ozonation of oxalic acid does not appear to increase the observed degradation rate and decreases rates at millimolar levels.