The phase behavior and critical parameters of the CO2 (1) + pentane (2) + acetone (3) ternary system with different compositions were determined experimentally, and their densities were measured in both the subcritical and supercritical (SC) regions. The isothermal compressibility (K-T) at some typical conditions 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, i.e, K-T is large and increases sharply. K-T also increases significantly as 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 composition, K-T has small values, and the effect of pressure on K-T is very small, even near the phase-separation point.