Abstract Contrary to the earlier claim that diphenyl sulfoxide ionizes in nearly 100% sulfuric acid to give a “diphenyl sulfidonium” ion, which can then return to the original sulfoxide when quenched with water, the 18O tracer experiment and a new set of cryoscopic determinations of a few sulfoxides have revealed that the dissociation of the sulfoxide takes place in a concentrated sulfuric acid in the following manner: (Remark: Graphics omitted.)