The outstanding production of sulfur dioxide at arc volcanoes is not satisfactorily explained by most of the geochemical models involving the magmatic redox conditions. It is commonly accepted that sulfur is transported mainly as sulfide (S^II−) and/or sulfate (S^VI) by mantle-derived melts, before being released as SO₂ and/or H₂S in volcanic emissions. We present the first experimental evidence that other S-species coexist in water-rich arc basalts, by using X-ray microspectroscopy of olivine-hosted melt inclusions. In particular, we propose a new model involving sulfite (S^IV) as the intermediate species dissolved in basaltic melts which results in highly efficient partitioning of sulfur into volcanic gas emissions at the origin of excess sulphur degassing observed at arc basaltic volcanoes.