Measurements of the abundance of anhydrite and pyrite veins and the distribution of alteration types in oceanic basement enable estimates of global mass and isotopic fluxes of sulfur during hydrothermal alteration of oceanic crust. Fluxes of 2.5 × 10 12 g/yr S (δ-34 S = 2.7%) from the volcanic section to seawater and 2.1 × 1012 g/yr of seawater S (21%) into the crust result in an insignificant change in the S content of altered ocean crust. These are clearly second-order fluxes compared to the riverine input of S to the oceans and the output of sedimentary pyrite. Most of the anhydrite that forms in the high-temperature axial convection cell must later be dissolved at lower temperatures during off-axis circulation. The mean δ-34 S of altered oceanic basement is 0.9%, only slightly changed from the primary value of 0.1%. Unless S is preferentially mobilized from 34S-rich sulfide deposits or mineralized zones, subduction of altered basaltic crust is unlikely to be the source of high δ34S values of arc volcanic rocks (∼4%), but could contribute to isotopic heterogeneities of S in the mantle.