It has long been recognized that melt inclusions (MIs) have the potential for providing insight into how magmas and their volatiles evolve during cooling and crystallization. We analysed H2O, CO2, S and Cl contents of olivine-hosted MIs in several eruptive products of Mt. Etna (Italy), together with their major element composition, and iron speciation (Fe3+/ƩFe). Using these data, we investigated fractional crystallization, degassing and redox conditions variations occurring during magmatic ascent. MIs have composition extending from volatile-rich picro-basaltic toward volatile-poor evolved trachybasalt. The picro-basaltic MIs are extremely enriched in H2O and CO2 (up to 5.9 and 0.58 wt.% respectively). S content in Etnean MIs ranging from some ppm up to more than 4000 ppm, whereas the oxygen fugacity (fO2, estimated using XANES Fe3+/ƩFe ratios and oxybarometers models) ranges from NNO-0.9±0.1 to NNO+1.6±0.2, with NNO being the Nickel-Nickel Oxide buffer [1]. Based on (i) S concentration and (ii) oxygen fugacity estimates in MIs, and using an empirical model predicting the variation of S partitioning between fluid and melt phases (DSfluid/melt) upon variations in P, T and fO2, our research suggests that during magmatic ascent in the Etnean system a fO2 decrease occurs, affecting the S behaviour. The empirical DSfluid/melt model coupled to MELTS code [2, 3] provides evidence of fO2 decrease during volatile exsolution and crystal fractionation of olivine, clinopyroxene, spinel and plagioclase, upon T and P decrease. The fO2 diminution is responsible for the increase in DSfluid/melt, and thus the decrease of S solubility in the melt and, consequently, for the increasing exsolution of S (which further enhances the reduction of fO2) from the magma before eruption. References [1] Gennaro, Paonita, Iacono-Marziano, Moussallam, Pichavant, Peters & Martel (2020), Journal of Petrology, 61, 10. [2] Ghiorso & Sack (1995), Contribution to Mineralogy and Petrology, 119, 197-212. [3] Smith & Asimow (2005), Geochemistry Geophysics Geosystems 6, Q02004.