The mineralogy, contents, and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes. Serpentinization of <1 Ma peridotites at Hess Deep occurred at high temperatures (200¿--400 ¿C) and low water/rock ratios. Oxidation of ferrous iron to magnetite maintained low fO2 and produced a reduced, low-sulfur assemblage including NiFe alloy. Small amounts of sulfate reduction by thermophilic microbes occurred as the system cooled, producing low-Δ34S sulfide (1.5? to -23.7?). In contrast, serpentinization of Iberian Margin peridotites occurred at low temperatures (~20¿--200 ¿C) and high water/rock ratios. Complete serpentinization and consumption of ferrous iron allowed evolution to higher fO2. Microbial reduction of seawater sulfate resulted in addition of low-Δ34S sulfide (-15 to -43?) and formation of higher-sulfur assemblages that include valleriite and pyrite. The high SO4/total S ratio of Hess Deep serpentinites (0.89) results in an increase of total sulfur and high Δ34S of total sulfur (mean≈8?). In contrast, Iberian Margin serpentinites gained large amounts of 34S-poor sulfide (mean total S=3800 ppm), and the high sulfide/total S ratio (0.61) results in a net decrease in Δ34S of total sulfur (mean≈-5?). Thus serpentinization is a net sink for seawater sulfur, but the amount fixed and its isotopic composition vary significantly. Serpentinization may result in uptake of 0.4-14¿1012 g S yr-1 from the oceans, comparable to isotopic exchange in mafic rocks of seafloor hydrothermal systems and approaching global fluxes of riverine sulfate input and sedimentary sulfide output. ¿ 1998 American Geophysical Union