The phase behavior and critical parameters of CO2 (1) + pentane (2) and CO2 (1) + acetone (2) were determined experimentally, and their densities were measured in both the subcritical and supercritical regions. The isothermal compressibility (K-T) was calculated using the density data. It is demonstrated that the density is sensitive to pressure as the pressure approaches the critical point of the mixtures, that is, K-T is large and increases sharply. K-T also increases significantly as the pressure approaches the dew point or the bubble point at other compositions close to the critical composition. However, K-T is very small and is not sensitive to pressure well above the dew point or the bubble point. When the composition is far from the critical one, K-T is fairly small and the effect of pressure on K-T is very limited, even near the phase separation point.