Sulfur contents and δ34S values of Somma-Vesuvius magmas are consistent with syneruptive, open-system degassing at temperatures of 800-850°C for Plinian pumices and 1100-1200°C 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 δ34S 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.