Equilibrium interfacial tension (IFT) between heavy oil and CO2 with the addition of C3H8 and/or n-C4H10 has been experimentally and theoretically determined. Experimentally, an axisymmetric drop shape analysis (ADSA) technique is used to measure both the dynamic and equilibrium IFTs between heavy oil and three pure solvents (i.e., CO2., C3H8, and n-C4H10) together with seven solvent mixtures. Theoretically, the Peng-Robinson equation of state (PR EOS) with a newly developed alpha function has been incorporated into a mechanistic parachor model to determine the equilibrium IFTs between heavy oil and pure solvents together with their corresponding mixtures. The addition of C3H8 and/or n-C4H10 into CO2 stream leads to an obvious reduction of IFT between heavy oil and CO2, although the degree of reduction depends upon the added amount of rich solvent(s). The mechanistic parachor model with the optimized parachor of the heavy oil and mass-transfer exponent provides a qualitative agreement with the measured equilibrium IFTs between solvent(s) and heavy oil in the liquid vapor phase region.