A new transparent microscale circulation-type high pressure equilibrium cell with on-line sampling was devised. With this apparatus, experimental solubility of molecularly complex species such as steroids(cholesterol, stigmasterol and ergosterol) and fatty acids(palmitic acid and stearic acid) in supercritical carbon dioxide(sc-CO₂) were measured. Also, to find an appropriate substance for enhancing both the polarity and the solubility power of the sc-CO₂ solvent, we arbitrarily selected three polar substances such as acetone, methanol and water and the effect of these cosolvents on the solubility of solutes in sc-CO₂ are examined. The supercritical phase equilibrium data of solute-cosolvent-sc-CO₂ systems were quantitatively correlated using a new equation of state based on the lattice fluid theory incorporated with the concept of multibody interaction. We found that the addition of tracer amount of acetone or methanol to so-CO₂ enhances the solubility of all solutes about thirty to sixty times when compared with the case of pure sc-CO₂. However, for the case of cosolvent water, no further enhancement of the solubility of solutes was realized. Also, the versatile fittability of the equation of state proposed in this work was demonstrated with the newly measured ternary supercritical equilibrium data.