Abstract. Powders of four synthetic glasses of volcanic composition, a silica glass and crystalline quartz were equilibrated with SO₂ to study the speciation of S sorbed onto their surface. These samples mimic the aerosols injected into the atmosphere during volcanic eruptions. Volcanic sulfur is known to globally affect the Earth's climate with an opposite effect to CO₂. However, absorption on ashes may reduce the amount of sulfur entering the stratosphere. S K-edge micro-XANES (μXANES) spectra and μXRF maps were collected at the LUCIA beamline (SOLEIL) at the SLS (Switzerland). When photoreduction is minimized, SO₂ is sorbed mostly as sulfates moieties. The sorption of S is controlled by the surface structure of the powders probed. Presence of defects, non-bridging oxygens and network-modifiers (alkali and alkali-earths) enhance S-sorption as sulfate moieties onto the powders surface. Therefore, the quantity of S released to the atmosphere is highly dependant on the type of ash produced during eruptions that help to better model the climatic impact of volcanic S.