Sulfonic acid is a strong acid of the formula H-S(=0)(=O)-OH or H₂SO₃. Its anion and its salts are called sulfonates.

The sulfonic acid or sulfonate functional group -S(=0)(=O)-OH or -SO₃H is found in many chemical compounds, e.g. certain detergents and dyes as well as in strongly acidic cation exchange resins.

See also

• Sulfuric acid H₂SO₄