Fluid inclusions in quartz grains from five samples of high-grade rocks (two paragneisses, an amphibolite, a mafic gneiss and a tonalite dike) from the 2.7 Ga Kapuskasing structural zone (KSZ), Ontario, were examined with petrographic, microthermometric and laser Raman techniques. Three types of fluid inclusions were observed: CO2-rich, H2O-rich and mixed CO2-H2O. CO2-rich fluid inclusions are pseudosecondary or secondary in nature and are generally pure CO2; a few contain varying amounts of CH4H2O-rich fluid inclusions are secondary in nature, contain variable amounts of dissolved salts, and generally contain daughter crystals. Mixed CO2-H2O fluid inclusions occur where trails of H2O-rich inclusions intersect trails of CO2-rich inclusions. Isochores for high density (p=1.03 g/cm3) pseudosecondary, pure CO2 inclusions intersect the lower pressure portion of the estimated P-T field for high-grade metamorphism, implying that pure CO2 was the peak metamorphic fluid. The variable CH4 content of CO2 inclusions within graphite-bearing samples suggests that CH4 was introduced locally after the formation of the CO2 inclusions; however the origin of the CH4 remains problematic. An aqueous fluid clearly penetrated the gneisses after the peak metamorphism (during uplift/erosion), forming secondary inclusions and contributing to the minor retrogressive hydration observed in these rocks. The presence of the pseudosecondary, high-density CO2 inclusions in quartz crystals in the KSZ rocks constrains the uplift/ erosion path for the KSZ to one of simultaneous decrease in pressure and temperature.