连二硫酸（H₂S₂O₆）是一种只能在溶液中存在的化合物。

连二硫酸是一种较稳定的强酸。室温下，稀的连二硫酸溶液较稳定。溶液被浓缩或者受热时，缓慢歧化分解为硫酸和二氧化硫：

连二硫酸中的硫元素氧化态+5，理因能够参与许多氧化还原反应。但实际上它与多数氧化剂（例如高锰酸钾、氯气、次氯酸钠）和一般的还原剂都不发生作用，这可能动力学原因造成的。

钠汞齐可将连二硫酸盐还原为亚硫酸盐或连二亚硫酸盐：

Dithionic acid, H₂S₂O₆, is a chemical compound known only in solution.

Dithionic acid is dibasic and salts called dithionates are known. No acid salts have been discovered. All dithionates are readily soluble in water. They are mild oxidizing and mild reducing agents. The structure of dithionate ion is like ethane, but two SO₃ groups adopt an almost eclipsed conformation. The S—S length is 2.15 Å; S—O bonds are rather short with bond length of 1.43 Å.