The term commonly refers to peroxomonosulphuric(VI) acid, H2SO5, which is also called permonosulphuric(VI) acid and Caro's acid. It is a crystalline compound made by the action of hydrogen peroxide on concentrated sulphuric acid. It decomposes in water and the crystals decompose, with melting, above 45°C. The compound peroxodisulphuric acid, H2S2O8, also exists (formerly called perdisulphuric acid). It is made by the high-current electrolysis of sulphate solutions. It decomposes at 65°C (with melting) and is hydrolysed in water to give the mono acid and sulphuric acid. Both peroxo acids are very powerful oxidizing agents. See also sulphuric acid (for structural formulas).