Peroxydisulfuric acid is an inorganic compound with a chemical formula (HO3SO)2.

[1] This oxoacid features sulfur in its +6 oxidation state and a peroxide group.

[2] The acid is prepared by the reaction of chlorosulfuric acid with hydrogen peroxide:[3] Another method is the electrolysis of moderately concentrated sulfuric acid (60-70%) with platinum electrodes at high current density and voltage: Peroxydisulfuric acid is a precursor to several salts including sodium peroxydisulfate, potassium peroxydisulfate, and ammonium peroxydisulfate.

These salts are used to initiate the polymerization of acrylonitrile, styrene, and related monomers.

This application exploits the tendency of the peroxydisulfate anion to undergo homolysis to produce radicals.