Magnesium sulfite is the magnesium salt of sulfurous acid with the formula MgSO3.

Its most common hydrated form has 6 water molecules making it a hexahydrate, MgSO3·6H2O.

When heated above 40 °C (104 °F), it is dehydrated to magnesium sulfite trihydrate, or MgSO3·3H2O.

[1] The anhydrous form is hygroscopic, meaning that it readily absorbs water from the air.

Solubility tables of MgSO3 hydrates PDF: [2]