Typically, the addition of a non-adsorbing polymer to a system containing colloidal particles causes a phase separation in a mechanism known as depletion flocculation. Recent experiments have shown that the addition of polystyrene to a mixture of asphaltenes and toluene causes phase separation into two liquids.(1) In this paper, this effect is modeled using the perturbed-chain form of statistical associating fluid theory (PC-SAFT) equation of state, demonstrating agreement with the experimental data. The effects of the temperature, pressure, and polystyrene mean molecular weight on the mixture phase behavior are investigated. The phase behavior of the system was not sensitive to pressures up to 1500 atm; however, increasing the temperature or reducing the mean molecular weight of polystyrene caused the one-phase region to expand. The paper demonstrates that a solution model with rigorous physics is able to capture the phase behavior that is typically described as a depletion flocculation mechanism in the colloidal literature.