Techniques have been developed to experimentally and theoretically determine phase behaviour and viscosity reduction of CO2-heavy oil systems at high pressures and elevated temperatures. Experimentally, vapour-liquid phase boundaries (i.e., saturation pressure lines) and the swelling factors are measured by conducting PVT tests at pressures up to 11094.0 kPa and temperatures up to 362.75 K, respectively. The viscosity of CO2-saturated heavy oil is measured at 319.15 K. Theoretically, the heavy oil sample is respectively characterized as a single- and multi-pseudocomponent(s). An exponential distribution function is used to split the plus fraction of heavy oil up to C105+, while a logarithm-type lumping method is used to group the single carbon numbers (SCNs) into multiple pseudocomponents. Then, the Peng-Robinson equation of state (PR EOS) coupled with the modified alpha function is applied to quantify the phase and volumetric behaviour of the CO2-heavy oil systems. The binary interaction parameters (BIPs) for CO2-pseudocomponent(s) pair are tuned to match the measured saturation pressures. Compared with the characterization scheme of treating heavy oil as a single pseudocomponent, the absolute average relative deviation (AARD) for the predicted saturation pressures can be improved from 5.27% to 4.56% by characterizing the heavy oil as six pseudocomponents. With the optimum BIPs, the swelling factors are predicted by the PR EOS with and without the volume translation technique, respectively. It is found that the introduction of the volume shift to each (pseudo)component in the PR EOS is able to provide more accurate prediction in both characterization schemes with AARD of 1.88% (oil as a single pseudocomponent) and 1.39% (oil as six pseudocomponents), respectively.