Abstract Sulfur contents and 34S values of Somma-Vesuvius magmas are consistent with syneruptive, open-system degassing at temperatures of 800850C for Plinian pumices and 11001200C for lavas. The extent of degassing appears to be greater in lavas than in pumices. The key parameter controlling the 34S/32S ratio of Somma-Vesuvius volcanics is the average magma oxidation state, which generally varies from 0.85 to 1.20 �� NNO units for lavas and from 1.20 to 1.40 �� NNO units for pumices. Consequently, S contents and 34S values of magmas constitute a potentially valuable tool in estimating their average redox conditions. The results of this study may help in risk mitigation when the Vesuvius magmatic system evolves toward eruptive conditions.