Phase relations among sillimanite, cordierite, garnet, biotite, and hypersthene from regionally metamorphosed pelitic gneisses were determined from petrographic studies and the chemical compositions of 46 ferromagnesian minerals and 18 bulk rocks. The compatible mineral associations including quartz, feldspar, and opaque oxides are cordierite-sillimanite, cordierite-garnet-sillimanite, cordierite-garnet-biotite, cordierite-garnet-hypersthene, cordierite-biotite-hypersthene, cordierite-biotite, garnet-biotite, garnet-biotite-hypersthene, and biotite-hypersthene. The assemblages were graphically analyzed using A–F–M diagrams derived from compatibility tetrahedra by successive projections through the common phases quartz, alkali feldspar, plagioclase, magnetite, and ilmenite; this results in the subtraction of excess components such that A = Al2O3 − K2O − Na2O − CaO, F = FeO − Fe2O3 − TiO2, and M = MgO. Variations in the positions of the three-phase triangles defined by cordierite, garnet, and biotite in the A–F–M system are due to systematic variations of F: M ratios for these minerals and reveal that the external conditions of metamorphism were variable over the rock sequence studied. Partitioning of elements among coexisting minerals and field evidence indicate that equilibrium was reached at constant temperature in the gneisses around Gananoque Lake; possible variations in load pressure were inadequate to cause the observed variations in F/M. A correlation between the Fe+2/(Fe+2 + Mg) of coexisting ferromagnesian silicates and the oxidation ratios (2Fe2O3/(2Fe2O3 + FeO)) of respective rocks suggests that the mineralogical variations in F/M are a function of oxygen partial pressure. Increased oxygen pressures would give rise to magnetite at the expense of the ferromagnesian silicates, which would consequently become enriched in the magnesium end-members. It is further proposed that the equilibrium partial pressures of oxygen and water were interdependent in any small volume of pelitic gneiss during metamorphism, and that was the independent variable.