Hydrothermal fluids collected from the ASHES vent field in 1986, 1987, and 1988 exhibit a very wide range of chemical composition over a small area (~60 m in diameter). Compositions range from a 300 ¿C, gas-enriched (285 mmol/kg CO2), low-chlorinity (~33% of seawater) fluid to a 328 ¿C, relatively gas-depleted (50 mmol/kg CO2), high-chlorinity (~116% of seawater) fluid. The entire range of measured compositions at ASHES is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust, followed by partial segregation of the vapor and brine phases. Other mechanisms proposed to produce chlorinity variations in hydrothermal fluids (precipitation/dissolution of a chloride-bearing mineral or crustal hydration) cannot produce the covariation of chlorinity and gas content observed at ASHES. There is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation, in which gases are partitioned according to Henry's law and all salt remains in the liquid phase. Significant enrichments in silica, lithium and boron in the low-chlorinity fluids over levels predicted by the model are attributed to fluid-rock interaction in the upflow zone. Depletions in iron and calcium suggest that these elements have been removed by iron-sulfide and anhydrite precipitating at some time in the history of the low-chlorinity fluids. The distribution of low- and high-chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability. The relatively shallow depth of the seafloor (1540 m) and the observed chemistry of ASHES fluids are consistent with phase separation in the sub-critical or near-critical region. ¿ American Geophysical Union 1990