The Frontenac Axis is a southeasterly extension of the Grenville province of the Canadian Shield, which connects to the Adirondack Mountains in New York. A mineralogic and petrologic study of the high-grade pelitic gneisses in three sampled areas in the Westport and Gananoque map areas was undertaken to examine the dynamic nature of metamorphic and postmetamorphic history in the area. The mineral assemblages indicate pressures of 4–6 kbar, (4–6 × 105 kPa), temperatures of 675–800°C, and during peak metamorphism based on experimentally determined phase equilibria. Chemographic relations reveal that the highest grade rocks in southeastern Ontario (cordierite–orthopyroxene zone) are in the northeast corner of the Gananoque map area and in the area northwest of Gananoque. On the scale of a thin section, chemical equilibration has not been achieved as supported by chemical zoning, chemical heterogeneity, and absence of exchange equilibrium. On the scale of a rock, graphical analysis indicates that the continuous variation in the Mg/(Mg + Fe) ratio of ferromagnesian phases (assemblage: biotite + sillimanite + cordierite + garnet + quartz + alkali feldspar) is in response to temperature, which conceals any evidence of local gradients in . Different temperatures are recorded across a rock because of reequilibration during cooling. The direction of displacement of reactions that produce (1) the corona textures; (2) the chemical zoning associated with corona textures; and (3) the chemical zoning related to exchange reactions between adjacent phases, is consistent with a decrease in temperature, pressure, or both. Intermingling of incompatible assemblages and the widespread occurrence of six-phase AFM assemblages are due to disequilibrium produced by incomplete reaction during uplift and cooling.