Submarine hydrothermal vent fluid compositions may be controlled by peridotite-seawater or basalt-seawater reactions. Previous studies of slow-spreading ridges indicate that in addition to basalts, peridotites often play a prominent role in the construction of upper oceanic crust. Therefore the surface outcrop at a submarine hydrothermal vent field may not reveal the composition of the rock that hosts the reaction zone. We can, however, predict the compositional differences between ultramafic- and basalt-hosted vent fluids by using theoretical reaction path calculations. These calculations determine equilibrium fluid compositions and mineral assemblages, yielding synthetic hydrothermal vent fluid compositions that can be compared to analytical measurements. Synthetic vent fluid compositions created from basalt and seawater reactants at 350¿ or 400 ¿C and 500 bars are in close agreement with analytical measurements of end-member vent fluids from mid-ocean ridges. Twenty simulations at a 1:1 water to rock ratio using basalt compositions spanning the range of geochemical variability observed amongst mid-ocean ridge basalts yield vent fluid compositions with <20% variation in major element concentrations. We also performed 15 ultramafic-seawater simulations using dunite, lherzolite, and harzburgite compositions found in oceanic crust. All produced aqueous SiO2, K, and H2 concentrations that are distinct from the basalt-seawater calculations. These differing concentrations can be used to attribute analytical measurements of vent fluid compositions to ultramafic or basaltic reaction zones. ¿ 2000 American Geophysical Union