In this study, the oxidation of multiwalled carbon nanotubes (MWCNTs) sonicated and/or refluxed in acids (H₂SO₄/HNO₃) was investigated using a combination of high-resolution transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and ab initio computational methods. Sonication of the sample has the strongest effect, leading to the highest concentration of defects and carboxyl groups on the walls. Substantial correlations between treatment protocols, CNT size, and the types of chemical moieties are observed. Finally, based on experimental and computational results, we suggest the mechanism of the oxidation process for attaching the functional groups on the sidewalls.