The sonochemical decomposition of a low concentration of butyric acid was performed in an aqueous solution by use of 200 kHz ultrasound to discuss the reaction kinetics and molecular behavior during cavitation. Taking into account a Langmuir-type adsorption model, we propose a heterogeneous reaction kinetics model, which is based on the local reaction zone at the interface region of the cavitation bubbles, where the adsorption and desorption of butyric acid molecules from the bulk solution occur during bubble oscillation and then the existing molecules inside the local reaction zone are finally decomposed. To confirm our proposed kinetics model, the rates of decomposition were investigated as a function of the initial concentration of butyric acids in the different pH solutions. It was confirmed that our model could be reasonably applied to explain the obtained results and the pseudo rate constant (k) and the equilibrium constant (K) were able to be calculated: k is 8.0 microM min(-1) (pH 2) and 3.5 microM min(-1) (pH 10), and K is 5.7 x 10(-3) microM(-1) (pH 2) and 8.0 x 10(-3) microM(-1) (pH 10), respectively. By the analysis of the obtained K values, it was clear that the ionized organic acid molecules are relatively difficult to accumulate at the reaction zone, because of their lower hydrophobicity compared with that of the neutral ones. The results obtained in the sonochemical decomposition of benzoic acid were also able to be analyzed with the proposed kinetics model. In addition, we proposed an opinion toward the interpretation of a Langmuir-type adsorption model which has often been applied to explain heterogeneous reaction systems.