The one-, two-, and three-phase isochoric heat capacities (C VX ) of n-hexane+water mixtures near the lower critical line, including UCEP, have been measured with a high‐temperature and high‐pressure nearly constant‐volume adiabatic calorimeter. The measurements were made in the temperature range from 443K to 501K for 8 liquid and vapor isochores from 121.07kgm −3 to 377.5kgm −3 . The isochoric heat capacity jump (quasi-static thermograms supplemented by the sensor of adiabatic control) technique has been used to accurately measure the liquid–liquid–vapor (L–L–V) and liquid–vapor (L–V) phase transition parameters (T S , ρ S ). The expanded uncertainty of the heat-capacity measurements at 95% confidence level with a coverage factor of k=2 is estimated to be 0.06% for density (ρ), 15mK for temperature (T), and 2% to 3% for isochoric heat capacity (C V ). The liquid–gas critical temperatures,T C (x), and the critical densities,ρ C (x) (lower critical lines) for binary n-hexane+water mixtures were determined from the measured saturated properties (C VS ,T S , ρ S ) near the critical point. The temperature, density, and the pressure at the UCEP (T UCEP =496.13±0.2K, P UCEP =5.220±0.10MPa, ρ UCEP =263.92±2kgm −3 ) were extracted from the measured critical line and three-phase experimental data. For the first time we reported the temperature dependent behavior of C VX near UCEP.