Anhydrite has been identified as a phenocrystic phase in some silicic volcanic magmas, but it is not commonly described in plutonic rocks. Anhydrite-bearing magmas tend to form in arc environments and to contain hydrous, low-temperature, oxidised mineral assemblages. Phenocrystic anhydrite coexists with sulphur-enriched apatite and sometimes with pyrrhotite, in silicate melt that contains from 50 ppm to 1 wt% S, depending on temperature and f _O2 conditions. Vapour coexisting with anhydrite- and water-saturated magma may contain from a few tenths of a mole per cent to a few mole per cent sulphur gases (SO₂ and H₂S), with the exact composition and gas speciation depending on temperature and oxygen fugacity. Samples of one anhydrite-bearing magma, the 1991 Pinatubo dacite, have been experimentally crystallised to determine whether the magma retains its characteristic sulphur-rich mineral phases during solidification. Results show that anhydrite and sulphur-rich apatite are retained throughout crystallisation and vapour phase evolution. This suggests that anhydrite-bearing intrusive equivalents of the Pinatubo dacite should be present in arc plutonic complexes.