Sulfur in volcanic arc magmas is an important tool for monitoring volcanic hazards and strongly influences the formation of porphyry ore deposits. Despite its importance, the extent to which subduction influences the sulfur content of arc magmas is debated. In this talk, I use direct measurements of sulfur isotope ratios and sulfur valence state in volcanic glass melt inclusions from the Southern Cascades to show that sulfur in arc magmas is partially derived from the subducting plate. Using quantitative models of mantle melting, I demonstrate that the transfer of oxidized sulfur from the subducting plate into the sub-arc mantle wedge changes the oxidation state of sulfur in sub-arc mantle melts. Magma sulfur isotope ratios and estimates of oxidation state in the Southern Cascades are similar to those measured in arcs globally. Combined, these results suggest that the concentration and behavior of sulfur in arc magmas is strongly influenced by materials released from subducting plates.