Reaction path models of seawater-basalt interaction at mid-ocean ridges similar to those presented by Bowers and Taylor (1985) are extended to include sulfur isotopic exchange equilibria. The effects of variable contribution of sulfur from basalt and of high- and low-temperature reduction of sulfate in the fluid are considered. Results of the model calculations are compared with Δ34S analyses of disseminated sulfides in altered basalts, chimney sulfides, and fluids collected from mid-ocean ridge hot springs. The calculations indicate that under the physical conditions of water-rock interaction in mid-ocean ridge circulation systems, the overwhelming influence on the sulfur isotope signature of exiting fluids is basaltic sulfur. No trace of Δ34S enrichment by reduction of seawater sulfate is recorded in hot spring fluids unless the reduction occurs at high temperature (350 ¿C) or near the venting areas. A comparison of recent oxygen and hydrogen isotope data from several vent sites suggests that contributions of altered basalt and fluid with a significant history of low-temperature interaction with basalt can be qualitatively assessed. ¿ Americal Geophysical Union 1989