A highly precise and accurate vibrating U-tube technique was developed to determine the upper baric stabilities of liquid-vapor assemblages in the CO2-H2O system at high subcritical temperatures (~275–360 °C). The first step is to create an isobaric-isothermal, physically isolated and chemically homogeneous sample of “high-pressure” CO2-H2O fluid of known composition. Fluid pressure (P) is then lowered slowly at constant temperature. Pressure readings and matching values for τ (the period of vibration of the U-tube) are recorded at 0.1 or 0.2 MPa intervals. When the fluid begins to separate into two phases (liquid + vapor), a distinct inflection is observed in the trend of P vs. τ. Performing such experiments for fluid compositions at 0.05 mole fraction CO2 (XCO2) intervals in the range 0.05 ≤ XCO2 ≤ 0.40 at 300 °C produced a complete high-P liquid-vapor boundary curve for the CO2-H2O system at that temperature. Agreement with corresponding curves determined in previous studies ranges from poor to excellent.