A hydrothermal diamond anvil cell (HDAC) was used to heat natural H₂O+CO₂+NaCl fluid inclusions in quartz from two locations in the Adirondack Mountains. Heating in the HDAC allows regulation of external pressure so the total homogenization of the aqueous and carbonic phases at high temperature can be observed and recorded with minimal risk of decrepitation. The total homogenization temperature, the CO₂-clathrate melting temperature, and the homogenization temperature of the carbonic phase is used precisely to calculate the pressure of total homogenization (on the H₂O+CO₂+NaCl solvus) and the corresponding isochore. The equation of state of Duan et al (1995) was used to calculate the solvus pressure by an iterative technique of varying the mole fraction CO₂ and the solvus pressure with a given total homogenization temperature and NaCl:H₂O molar ratio. Bulk molar volume values of the calculated fluids were iterated into a mixing equation for the aqueous and carbonic phases at low temperature until the fluid compositions, predicted by both equations, were identical. Seven H₂O+CO₂+NaCl fluid inclusions from post-metamorphic, hydrothermal cavities near Moose River, NY yielded homogenization temperatures of 185-232°C and homogenization pressures of 863 to 1870 bars. Two H₂O+CO₂+NaCl fluid inclusions from a drill core at the Willsboro, NY wollastonite mine yielded homogenization temperatures of 310°C and 324°C and homogenization pressures of 3893 and 5333 bars, respectively. These inclusions are interpreted to have formed along a retrograde path characterized by early isobaric cooling and later isothermal decompression following Ottawan-age metamorphism.