Study of massive sulfide deposits at 21°N, East Pacific Rise (EPR) indicates that coexisting sulfide minerals are not in isotopic equilibrium. Sulfur isotope values of sulfide mineral separates range from 1.5 to 4.5 per mil and indicate isotopic disequilibrium. Isotopic fractionation between coexisting minerals is small (<0.5). Larger differences between samples can be attributed to temporal variations in the isotopic values of H2S in the venting fluids. Sulfur isotope values of anhydrite and barite indicate disequilibrium with coexisting sulfides and show that ambient sea-water sulfate, not the hydrothermal fluid, is the sulfur source for anhydrite and barite. Oxygen isotope geothermometry applied to anhydrite sulfate provides temperatures of 194 and 222 °C. Detailed petrographic study of mineral paragenesis shows that the initial hydrothermal precipitates form from a fluid with Fo2-Fs2 values that are below the pyrite-pyrrhotite boundary. High-iron wurtzite may initially precipitate within the pyrrhotite stability field, but most wurtzite co-precipitates with pyrite as the fluid evolves into the pyrite stability field by mixing with cold oxygenated sea water. Precious-metal values, with the exception of silver, are uniformly low, especially for the platinoid metals. Silver content ranges to 240 ppm. Silver is predominantly associated with zinc-sulfide phases and does not occur in significant amounts as a discrete silver-bearing mineral.