Based on the Boit theory on acoustic propagation in porous media, ultrasonic method to determine the saturation vapor pressure of CO 2 has been investigated . As a result the speed and the attenuation of ultrasonic wave are very sensitive to the phase transitions of CO 2 in a PV cylinder both with and without porous media. Moreover it is indicated that porous media might affected the saturation vapor pressure of CO 2 to some extent.