The composition of sphalerite in equilibrium with pyrite + hexagonal pyrrhotite promises to be a useful geobarometer (Scott and Barnes, 1971). Solvus isobars were accurately located by hydrothermally recrystallizing sphalerite + pyrite + hexagonal pyrrhotite in aqueous alkali halide fluxes at 2.5, 5, and 7.5 kb between 325 degrees and 710 degrees C. Sphalerite becomes progressively less FeS-rich with increasing pressure, but the position and slope of the solvus isobars in T-X projection are somewhat different from those calculated by Scott and Barnes (1971). At 2.5 and 5 kb the composition of sphalerite is independent of temperature below approximately 600 degrees C at 17.92 + or - 0.37 and 14.46 + or - 0.12 mole % FeS, respectively. The 7.5 kb isobar is convex toward the temperature axis. Calculated sphalerite compositions at 5 kb in which the effects of thermal expansion and compression were included are in good agreement with the measured values between 400 degrees and 700 degrees CSphalerites from massive Cu-Zn deposits which have been metamorphosed within the greenschist facies are usually heterogeneous indicating that equilibrium was not attained. However at higher metamorphic grades the sphalerite is homogeneous or nearly so and the sphalerite geobarometer can be applied.The change with pressure in the composition of hexagonal pyrrhotite in equilibrium with pyrite was recalculated and found to be less than that given by Toulmin and Barton (1964). Up to 5 kb the pressure coefficient of pyrrhotite composition ranges from +0.037 at % Fe kb (super -1) at 700 degrees C to +0.022 at % Fe kb (super -1) at 300 degrees C. Attempts to verify these calculations by experiment were unsuccessful.