Beneath Mount Pinatubo in the Philippines, a hot mafic melt ascended, releasing supercritical fluids rich in SO2into an overlying semisolidified dacitic magma. The SO2 was reduced to H2S in the cool, wet dacite, causing oxidation of this magma. H2S thus formed was initially precipitated in the dacite as sulfides, which were high in Cu, Cd, Zn, and Se/S, elements also introduced by the fluids. Continued influx of SO2 and oxidation of the dacite led to an increase in the S solubility of the melt, causing partial resorption of sulfide minerals. Further addition of SO2 then led to excess S, which, in part, was precipitated as anhydrite. High S contents and the oxidized nature of the eruption products were due to the conjunction of an overlying, cool dacitic magma and ascending hot mafic melt. The 1982 eruption products of El Chichón (Mexico) and those from the 1985 eruption of Nevado del Ruiz (Colombia) have features similar to Pinatubo, suggesting that these high-S magmas may have formed by a similar process.