Abstract Sonication induces complex chemical and physical phenomena in solution, which attracted great attentions to discover the mechanisms behind these phenomena. One of these phenomena is the pH drop in sonication, which was usually attributed to the formation of nitrate and nitrite acid in sonication. However, the pH drop detected by electrodes was questioned since the sonication could enhance proton transfer on the surface of pH electrode, resulting in a false pH drop. Therefore, it is necessary to initiate a tour study on this issue. In this paper, we firstly investigate two new factors, namely CO2 dissolution and temperature, which have the possibility to induce the pH drop in sonication. The results demonstrate that both factors play a minor role on the pH drop. Then we employ pH sensitive dyes to replace the questioned pH electrode to detect the change of proton concentration in sonication. The results indicate that the pH drop is accompanied with the protonation of fluorescein, indicating production of hydrogen ions in sonication. The quantitative analysis of proton production suggests that the pH drop in sonication is mainly caused by the formation of hydrogen ions, which clarify the reason for the pH drop in sonication.