This paper presents new phase equilibria compositions and density data for (1) a quaternary mixture of propane + pentane + octane + CO2 at temperatures from 310 K to 394 K and pressures from 3 MPa to 6.374 MPa. Additionally, Liquid densities for a ternary mixture of propane + pentane + octane at four temperatures and gas CO2 densities are included for comparison with Literature values. A single mixture load can be used to measure simultaneously phase equilibria compositions and densities at various pressures and temperatures. This can be accomplished using a uniquely designed pressure/volume/temperature-vapor/liquid/liquid equilibrium apparatus (PVT-VLLE) connected to a uniquely designed system for infinitesimal sampling to a gas cromatograph (GC) at high pressures and temperatures. Carbon dioxide density compares well with the published data of Vargaftik and the predicted densities by DDMIX. Saturated liquid hydrocarbon densities are within 0.5% of the densities predicted by the modified Rackett equation. For liquid mixtures containing CO2, the deviations from the Rackett equation are within +/-1%, Therefore we conclude that this methodology is suitable for determining volumetric and phase equilibria properties of multiphase systems with sufficient accuracy for most engineering applications.