El ácido sulfuroso o ácido trioxosulfúrico (IV) es un ácido mineral que se forma al disolverse el óxido de azufre (IV) en agua. Es un agente reductor potente y se puede usar por ejemplo para blanquear manchas en materiales dañados por cloro.Se puede formar cuando el óxido de azufre (IV) se disuelve en las gotas de lluvia, causando lluvia ácida.En el laboratorio se prepara añadiendo ácido clorhídrico a una disolución de sulfito de sodio en agua. Na2SO3 + 2 HCl → H2SO3 + 2 NaCl

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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. Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO2 molecule and the bisulfite ion, HSO−3. The intensities of the signals are consistent with the following equilibrium: SO2 + H2O ⇌ HSO−3 + H+ Ka = 1.54×10−2; pKa = 1.81. 17O NMR spectroscopy provided evidence that solutions of sulfurous acid and protonated sulfites contains a mixture of isomers, which is in equilibrium: [H–OSO2]− ⇌ [H–SO3]− When trying to concentrate the solution by evaporation to produce waterless sulfurous acid it will decompose (reversing the forming reaction). In cooling down a clathrate SO2 · 5,75 H2O will cristallize which decomposes again at 7 °C. Thus sulfurous acid H2SO3 cannot be isolated.

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