亚硫酸是化学式为“H₂SO₃”的无机化合物，实际上是二氧化硫的水溶液。真正的亚硫酸分子只在气态有发现，溶液中的尚未观测到。 其对应的正盐为亚硫酸盐，酸式盐为亚硫酸氢盐。

根据拉曼光谱的数据，二氧化硫的水溶液中存在的全部是二氧化硫分子及亚硫酸氢根离子，与下列平衡有关：

二氧化硫水溶液具有还原性。

经由二氧化硫水溶液漂白前后之橡皮塞

正浸泡在二氧化硫水溶液中的橡皮塞

Sulfurous acid (also sulphurous acid) is the chemical compound with the formula H₂SO₃. There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase. The conjugate bases of this elusive acid are, however, common anions, bisulfite (or hydrogen sulfite) and sulfite. Sulfurous acid is an intermediate species in the formation of acid rain from sulfur dioxide.