The amounts of sulfur released in explosive volcanic eruptions are often orders of magnitude Larger than those expected to result from the degassing of the erupted SiO2-rich magma, Experimentally measured fluid/melt partition coefficients of sulfur ranged from 47 under oxidizing conditions (where SO2 is the dominant sulfur species in the fluid) to 468 under reducing conditions (where H2S dominates). Therefore, a few weight percent of hydrous fluid accumulated in the top of a magma chamber may extract most of the sulfur out of the entire magma reservoir and generate sulfur-excesses upon eruption.