The Peng-Robinson equation of state is applied in this study to calculate the solubilities of solids in supercritical carbon dioxide and cosolvents. The mixture parameters of the equation of state are determined from a modified Huron-Vidal type mixing model with a volume correction term. The UNIFAC activity coefficient model is used in this work, and only a few group interaction parameters are needed in the determination of equation of state parameters for mixture calculations. The volume correction term is well-correlated in this research as a function of the molar volume of the solid molecule. Satisfactory results of the solubilities of solids in binary systems with supercritical carbon dioxide and in ternary mixtures with an additional cosolvent are shown in this work. This model yields satisfactory solubility results which are superior to those from the traditional van der Waals mixing model with empirically fitted binary parameters.