The isochoric heat capacity of two binary (CO2+n-decane) mixtures (0.095 and 0.178 mole fraction of n-decane) have been measured with a high- temperature, high-pressure, nearly constant volume adiabatic calorimeter. Measurements were made at nineteen near-critical liquid and vapor densities between 87 and 658 kg·m−3 for the composition of 0.095 mole fraction n-decane and at nine densities between 83 and 458 kg·m−3 for the composition of 0.178 mole fraction n-decane. The range of temperatures was 295 to 568 K. These temperature and density ranges include near- and supercritical regions. The measurements were performed in both one- and two-phase regions including the vapor + liquid coexistence curve. The uncertainty of the heat- capacity measurements is estimated to be 2% (coverage factor k=2). The uncertainty in temperature is 15 mK, and that for density measurements is 0.06%. The liquid and vapor one-(C′v1) and two-phase (C′V2, C″V2) isochoric heat capacities, temperatures (TS), and densities (ρS) at saturation were measured by using the well-established method of quasi-static thermograms for each filling density. The critical temperatures (TC), the critical densities ρC), and the critical pressure (PC) for the CO2+n-decane mixtures were extracted from the isochoric heat-capacity measurements on the coexistence curve. The observed isochoric heat capacity along the critical isochore of the CO2+n-decane mixture exhibits a renormalization of the critical behavior of CVX typical for mixtures. The values of the characteristic parameters (K1, K2), temperatures (τ1, τ2), and the characteristic density differences (Δ$$1, Δ$$2), were estimated for the CO2+n-decane mixture by using the critical-curve data and the theory of critical phenomena in binary mixtures. The ranges of conditions were defined on the T-x plane for the critical isochore and the ρ-x plane for the critical isotherm, for which we observed renormalization of the critical behavior for the isochoric heat capacity.