In the Granite Falls-Montevideo area, Minnesota, granulite facies mineral assemblages were collected from three major lithologic units—hornblende-pyroxene gneiss, garnet-biotite gneiss, and granitic gneiss. Mineral assemblages most commonly observed are: plagioclase-hornblende-orthopyroxene-clinopyroxene-magnetite-ilmenite-(biotite-quartz); quartz-plagioclase-biotite-garnet-orthopyroxene-(orthoclase); quartz-plagioclase-biotite-garnet-(orthoclase); quartz-plagioclase-microcline-(garnet-hematite); and quartz-plagio-clase-microcline-biotite-(garnet-hematite-rutile). Partial analyses of isomorphous phases from the hornblende-pyroxene gneiss and the garnet-biotite gneiss were determined with an electron microprobe. Negligible compositional variation within single grains and of a particular mineral within a given specimen, regular distribution of Fe and Mg between coexisting hornblende, ortho-pyroxene, and clinopyroxene, obedience of the mineral assemblages to the Gibbs Phase Rule, and lack of empirically determined incompatible phases indicate a close approach to chemical equilibrium during metamorphism. Plots of coexisting biotite, garnet, and orthopyroxene on an appropriate phase diagram result in some crossing tie lines which cannot be adequately explained by temperature or pressure differences, but suggest that H2O and/or O2 were not perfectly mobile components during metamorphism. This is also indicated by interlayering of hornblende assemblages with pyroxene assemblages and by different iron oxide phases in essentially the same mineral assemblage. Textural and chemical relationships of retrograde metamorphic assemblages suggest that some retrograde reactions are a result of cooling following the granulite-facies metamorphism, but that others may have resulted from recrystallization during a stage of thermal metamorphism that is reflected in the potassium-argon and rubidium-strontium biotite ages of the metamorphic rocks.