The phase behavior and critical parameters of (carbon dioxide + 1-bromobutane), (carbon dioxide + 1-chlorobutane), and (carbon dioxide + 1-methylimidazole) have been determined using a high-pressure variable-volume view cell, and their densities have also been measured in sub- or supercritical regions. The isothermal compressibility (K(T)) is calculated from the density of the binary mixtures. The transition points, bubble point, dew point, and critical point, have been measured with concentrations of organic solvent mole fractions from (0.0102 to 0.1495), temperatures from (308.2 to 337.4) K, and pressures from (6.21 to 19.04) MPa. It is demonstrated that the density is sensitive to the pressure as the pressure approaches the critical point of binary mixtures; that is, K(T) is large and increases significantly. K(T) also increases sharply when the pressure approaches the dew point or bubble point at other compositions near the critical composition. When the pressure is Much higher than the phase transition pressure or the composition is far from the critical composition, K(T) is rather small, and the effect of pressure on K(T) is fairly limited. The phase boundary data of the binary mixtures call be correlated well by the Peng-Robinson equation of state (PR EoS) with two binary parameters.