The hybrid steam-solvent injection has been considered as a promising technique for enhancing heavy oil/bitumen recovery，while its main mechanisms including the heat transferred and dissolution of solvents （e.g.， CH₄，C₂H₆，C₃H₈， C₄H₁₀，CO₂，N₂，and DME）into heavy oil/bitumen to reduce its viscosity and swell it are closely related to the phase behaviour of the solvents/water/heavy oil systems. To allow the seamless integration with the existing reservoir simulators， the traditional cubic equations of state（i.e.，SRK EOS and PR EOS） have been modified and improved to accurately quantify the phase behaviour and physical properties of the aforementioned systems under equilibrium and nonequilibrium conditions. Firstly，a huge database has been built to develop the corresponding alpha functions by minimizing the deviation between the measured and calculated vapour pressures for water as well as non-hydrocarbon and hydrocarbon compounds available from the public domain. Such obtained alpha functions are further validated with enthalpy of vaporization for pure substances， and then the reduced temperature has been optimized and the eccentric factor has been redefined. Finally，a pressure-implicit strategy has been developed to optimize the binary interaction parameters（BIPs）by treating heavy oil as one pseudocomponent （PC） or multiple PCs. Also，the contributions of each solvent to the aforementioned systems have been compared and analyzed within a consistent and unified framework. In addition to new alpha functions for hydrocarbons and water， respectively，the reduced temperature is found to have its optimum value of 0.59 for the two equations of state （EOSs），while 0.60 is recommended for practical use. Such improved EOSs have been further employed to reproduce the experimentally measured multiphase boundaries （or pseudo-bubble-point pressures）， density，viscosity，（mutual）solubility，and preferential mass transfer for the aforementioned mixtures under equilibrium and nonequilibrium conditions. The swelling effect for the heavy oil can be enhanced due to the addition of C₃H₈ and/or C₄H₁₀ or their mixtures into the CO₂ stream. Due to the existence of water，isenthalpic flash leads to more accurate quantification of multiphase boundaries and physical properties for the hybrid solvent-thermal processes. Each component of a binary or ternary gas mixture is found to diffuse preferentially into heavy oil at high pressures and elevated temperatures in the absence and presence of porous media，while each of them is found to exsolve differently from gas-saturated heavy oil under nonequilibrium conditions.