The effect of H2O on sulfide melting temperatures has been investigated in the FeS-PbS-ZnS system at 1.5 GPa, revealing that the addition of H2O results in a 35°C drop in melting temperature from 900° to 865°C. In addition to the melting point depression, the solubility of H2O is confirmed by the presence of vesicles in the quenched melt. No oxide phases were present in any of the run products, ruling out oxidation as a cause of the melting point depression. Confirmation of the solubility of H2O in sulfide melts is consistent with the recent suggestion by Mungall and Brenan (2003) of a magmatic origin for halogen-rich alteration associated with magmatic sulfide ore deposits, as the hydrous component of the alteration may similarly originate in the fractionating sulfide melt. Anatectic sulfide melts could be expected to contain more H2O than magmatic sulfide melts, owing to the lack of a parental silicate melt that buffers the H2O content of magmatic sulfide melts. Fluids expelled from the cooling anatectic melts, such as that present during granulite facies metamorphism of sulfide deposits at Broken Hill, Australia, may have been responsible for associated retrograde hydrothermal alteration.