Thermometers based on sulfur and oxygen isotopic compositions of anhydrite, pyrrhotite, titanomagnetite, and plagioclase crystals from fresh pumices of the 1982 eruptions of El Chichón Volcano indicate a pre-eruption temperature of 810 ± 40°C, confirming textural evidence that the anhydrite precipitated directly from the melt. The isotopic composition of sulfate leached from fresh ashfall samples shows it to be a mixture of anhydrite microphenocrysts and adsorbed sulfate derived from oxidized sulfur (SO₂) in the eruption plume. The leachate data show no evidence for rapid oxidation of significant amounts of H₂S in the eruption cloud even though the fugacity ratio of H₂S/SO₂ in the gas phase of the magma was >400. This may indicate kinetic inhibition of H₂S to SO₂ conversion in the eruption cloud. Prior to eruption, the magma contained an estimated 2.6 wt. % sulfur (as SO₃). The estimated δ ³⁴S of the bulk magma is 5.8‰. Such a high value may reflect assimilation of ³⁴S-enriched evaporites or the prior loss of ³⁴S-depleted H₂S to a fluid or gas phase during formation of a small prophyry-type hydrothermal system or ore deposit. In either case, the original magma must have been very sulfur rich. It is likely that the initial high sulfur content of the magma and at least some of its ³⁴S enrichment reflects involvement of subducted volcanogenic massive sulfides deposits during Benioff-zone partial melting. Isotopic data on mineralized, accidental lithic fragments support the possible development of a porphyry-type system at El Chichón.