The equilibrium solubilities of benzoic acid (BA), salicylic acid (SAL), and acetylsalicylic acid (ASA) were determined in binary (solute + CO2), ternary (two solutes + CO2), and quaternary systems (three solutes + CO2) at temperatures ranging from (308 to 328) K and pressures ranging from (10.1 to 28.0) MPa. Solubility data were obtained using a dynamic approach with a simple and reliable apparatus. Polar mixed-solid solute systems demonstrated solubility enhancements, which were consistent with the entrainer effect. In all the polar ternary systems studied, at least one component exhibited solubility enhancements. In the polar quaternary system studied, the solubility of each component increased in comparison to each binary system; the solubility of ASA, SAL, and BA was enhanced up to 484%, 248%, and 43%, respectively. The high solubility enhancements observed in our study indicate that solute-solute interactions are significant in the supercritical fluid (SCF) phase. The solubility enhancements observed in the polar mixed-solid solute systems studied resulted in a decrease in selectivity of SCF CO2. However, in a quaternary system consisting of BA, SAL, and fluoranthene (FLU), the selectivity of SCF CO2 for SAL versus FLU increased by a factor of 2.7 due to specific solute-solute interactions. This study showed that solute-solute interactions in mixed solid solute systems can result in an increase in the solubility of solutes and also the selectivity of SCF CO2.