Studies on the thermophysical properties of H2O/CO2 mixtures for supercritical conditions, typical for a promising power generation system are far fewer than those for typical conditions of CO2 capture and storage (lower temperatures and pressures). In the previous heat transfer studies, we have setup a high-temperature and high-pressure apparatus. Here, we have extended it to a multifunction apparatus to enable the measurements of both the isobaric heat capacity and density that are important for the understanding and prediction of heat transfer behaviors, besides the design of the power generation system. For the experimental conditions, the pressure is 24 MPa, the temperatures range from 300 °C to 410 °C, and the CO2 mass fractions are 10%, 15%, and 18.5%. The isobaric heat capacities have been measured using the flow calorimeter method. The expanded relative uncertainty is 8.2% for temperatures beyond ±4 °C from the pseudocritical point and is 18.2% near the pseudocritical point. The densities were obtained from the measurements of the pressure drops with an expanded relative uncertainty of 4.8%. These two methods were validated by supercritical pure water experiments. The isobaric heat capacity and density data given in this work, as well as our previous heat transfer data, are self-consistent regarding the pseudocritical temperatures of supercritical H2O/CO2 mixtures.