A circulation apparatus for simultaneously measuring the solubilities of solids in supercritical fluids and the densities of supercritical phases has been constructed. The apparatus can be used at pressures up to 20 MPa. The solubilities of salicylic acid in supercritical carbon dioxide with ethanol cosolvent and the densities of the fluid phase have been measured at 308.15 K and 318.15 K and at pressures up to 16 MPa. The concentration of ethanol in the vapor phase was in the range from 0 to 7 mol %. The effects of temperature, pressure, and cosolvent concentration on the solubility and the density have been investigated. The results show that the solubility of salicylic acid in supercritical carbon dioxide increases markedly with the concentration of the cosolvent. The solubility is a strong function of vapor-phase density when the concentration of the cosolvent is lower, and is nearly independent of the density at higher cosolvent concentrations. The interaction between the solute and the mixed solvent is very strong, especially through hydrogen bonding. The density of the fluid phase increases considerably with pressure and the concentration of the cosolvent.