2,4-dichlorophenoxyacetic acid (2,4-D) undergoes efficient degradation when an O-2- or Ar-saturated aqueous solution is subjected to high-frequency (640 kHz) sonolysis. 2,4-dichlorophenol, hydroquinone, and catechol are major reaction intermediates common to various experimental conditions. The similarities between the reaction intermediates of sonolytic and radiolytic reactions indicate (OH)-O-. radical as the primary reactive species responsible for 2,4-D degradation. Very little 2,4-D degradation occurs if the sonolysis is carried out in the presence of the (OH)-O-. radical scavenger tert-butyl alcohol, also indicating that little or no pyrolysis of the compound occurs. Most of the 2,4-D eventually forms oxalic acid, which, unlike other (OH)-O-.-mediated oxidation methods, is not easily mineralized with high-frequency ultrasound.