Ultrasonication is a common method for dispersing nanoparticles and colloids. We have found that, under certain conditions, unintended sonochemical reactions can be initiated by the incident ultrasonic energy, yielding unwanted byproducts. In this work, we determined that methyl hydroperoxide can be produced by an autoxidation chain reaction when ultrasonicating polar aprotic solvents containing methyl groups. Methyl radicals were detected during ultrasonication by their interaction with lucigenin, which emits sonochemiluminescence. A colorimetric triiodide test was used to confirm the presence of a hydroperoxide. The concentration of methyl hydroperoxide as a function of the ultrasonication time was measured by titration with NaOH. When above the critical coagulation concentration, this sonochemical byproduct collapses the electrical double layer, disrupting the dispersion stability and lowering the dispersion limits. This is significant when developing ultrasonication processes for dispersion of nanoparticles and colloids. There are no other examples of sonochemically initiated solvent autoxidation destabilizing single-walled carbon nanotube dispersions reported in the literature.