Ultrasonication is a necessary process to make single-walled carbon nanotubes (SWNTs) soluble in aqueous solution with surfactants such as sodium dodecyl sulfate (SDS). However, an understanding of the sonication effects on the electronic and optical properties of SWNTs in aqueous solution is still lacking. Here, we have observed that sonication-induced pH changes suppress the optical transitions of the first interband transition pair (S-11) in the density of states of semiconducting SWNTs while other possible intermediates induced by sonication contribute less significantly to the observed spectral changes without the involvement of sonication-induced pH decrease. The suppressed S-11 peaks can be restored by adding basic solution, suggesting that the lattice structure of SWNTs is undisrupted by the sonication used here. The absorbance of S-11 peaks shows a nearly linear relationship with sonication-induced pH changes in the narrow pH range of 5.2 and 6.1. The results indicate that SDS-encased SWNTs may be used as an indicator for sonolysis-related applications.