Sphalerite (sp) and hexagonal pyrrhotite (po) were recrystallized in a eutectic LiCl-KCl salt flux at 450 degrees to 750 degrees C and 1 to 6 kbars. The activity of FeS (a Fes ), controlled by the composition of excess pyrrhotite, varied from 0.5 to 1.0. Our data show that FeS activity-sphalerite composition relationships for any pressure are independent of temperature, i.e., the sphalerite composition (X sp Fes is a function of the FeS activity only, consistent with results of earlier investigations at low pressure.Plots of FeS activity vs. X sp Fes show that da Fes /dX sp Fes is a constant (defined as gamma sp Fes ), which increases with pressure and is also independent of temperature for the conditions of our experiments. The pressure dependence of gamma sp Fes as given by the expression: P(kbars) = 15.040 log gamma sp Fes - 2.672 (+ or -0.5 kbars) constitutes a geobarometer for rocks containing the trivariant assemblage sphalerite + hexagonal pyrrhotite. Although gamma sp Fes is independent of temperature for the range of a Fes = 0.5 to 1.0, a rough estimate of temperature is necessary to permit calculation of a Fes and hence gamma sp Fes . For example, sulfide ores of the North mine, Broken Hill, Australia, where N po Fes = 0.966 and X sp Fes = 0.201: at 650 degrees C, P = 6.6 kbars; and at 800 degrees C, P = 6.7 kbars. Pressures calculated from coexisting sphalerite and hexagonal pyrrhotite, at 650 degrees C, from the New Broken Hill Consolidated mine; the C lode and the Zinc Corporation mine; and the Pinnacles mine (12 km southwest of the main Broken Hill lode), indicate pressures of 6.8, 5.5, 6.2, and 6.3 kbars, respectively. These pressures are in excellent agreement with estimates based on silicate assemblages in adjacent host rocks.