Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO2 molecule and the bisulfite ion, HSO−3.

[2] The intensities of the signals are consistent with the following equilibrium: 17O NMR spectroscopy provided evidence that solutions of sulfurous acid and protonated sulfites contain a mixture of isomers, which is in equilibrium:[3] Attempts to concentrate the solutions of sulfurous acid simply reverses the equilibrium, producing sulfur dioxide and water vapor.

[4] However, the molecule has been detected in the gas phase in 1988 by the dissociative ionization of diethyl sulfite.

[6] Aqueous solutions of sulfur dioxide, which sometimes are referred to as sulfurous acid, are used as reducing agents and as disinfectants, as are solutions of bisulfite and sulfite salts.

They are oxidised to sulfuric acid or sulfate by accepting another oxygen atom.