Propagation of an ultrasonic wave in a water generates the cavitation bubbles, which behaving like microreactors are the center of a high-energy phenomenon leading to destruction of organic pollutants. In this work, a comparative study conducted with chlorobenzene and 4-chlorophenol as models for hydrophobic and hydrophilic substrates, respectively, clearly shows the two sites where the sonochemical reactions can occur. As ultrasonic frequency may alter noticeably the reaction rates, the experiments were conducted at 20 and 500 kHz. The dechlorination yields were found to be higher with the 500-kHz ultrasonic source than with the common 20-kHz probe. For chlorobenzene (ClBz), the reaction proceeds expeditiously, chlorine is quantitatively recovered as chloride ions, and 44% of the carbon atoms are recovered as CO and CO 2 . The rate of disappearance of starting material, Clrelease, and intermediate and final products provide some evidence of a reaction occurring thermally inside the cavitation bubble. 4-Chlorophenol (4-ClPh) degradation occurs at a lower rate. Clrelease and formation of hydroxylated intermediates are evidence of a two-step reaction involving the • OH radical outside the cavitation bubble. The consequences are important for water containing both 4-ClPh (0.5 mM) and ClBz (0.5 mM). ClBz degrades first; 4-ClPh transformation starts only when ClBz concentration reaches a low level (0.02 mM). The sonochemical treatment appears to be particularly efficient for the destruction of volatile chloroaromatic hydrocarbons (1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,3,5-trichlorobenzene, 1-chloronaphthalene).