In this paper, techniques have been developed to determine multiphase boundaries of solvent(s)/heavy oil mixtures at high pressures and elevated temperatures in pressure-temperature (P-T), enthalpy-temperature (H-T), and pressure-enthalpy (P-H) phase diagrams, respectively. Theoretically, the Peng-Robinson equation of state (PR EOS) incorporated with a new alpha function as well as the previously developed enthalpy calculation algorithm are used to predict the multiphase boundaries of the solvent(s)/heavy oil systems by characterizing the heavy oil as a single pseudocomponent. The PR EOS associated with the newly developed alpha function is found to be accurate in reproducing the experimentally measured VL1L2 (V represents the vapour phase, L1 denotes the high density hydrocarbon-rich liquid phase, and L2 refers to the low density CO2 liquid phase) three-phase boundary pressures with an overall absolute average relative deviation (AARD) of 2.01% and maximum average relative deviation (MARD) of 3.54%, respectively. In addition, the VL1L2 three-phase boundaries are expanded and tended to move toward the region with higher temperatures and lower pressures in the P-T phase diagram with the addition of either C3H8 or n-C4H10 to CO2/heavy oil systems compared to those of only CO2 exists in heavy oil systems. The enthalpy changes rapidly with variation of temperature in both H-T and P-H phase diagrams within the three-phase region. Either two-phase (VL1) or three-phase (VL1L2) isenthalpic flash calculations can be performed straightly to determine phase fractions and compositions together with system temperature without conducting the stability analysis for a provided solvent(s)/heavy oil mixture at a constant enthalpy and pressure with the constructed H-T phase diagram.