Hydrothermal fluids venting at 217°C from sulfide mounds have δ 3 4 S H 2 S = +7.8 ± 0.4, δ 1 8 O H 2 O = +0.43 ± 0.06, and δD H 2 O = -0.7 ± 0.3. The Escanaba Trough vent fluids have relatively low δ 1 8 O H 2 O values compared to those at several other mid-ocean ridge sites. Massive sulfide samples from various localities have δ 3 4 S s o l i d ranging from -0.7 to + 11.6. Isotopic ratios of oxygen, hydrogen, and sulfur are reproduced by simulated equilibrium isothermal closed-system reactions with sediments at relatively high cumulative water to rock ratios; they are reproduced by simulated closed-system reactions with basalt, or by open-system reactions with either sediment or basalt.