A new instrument is presented for measuring complex adiabatic compressibility and adiabatic pressure‐induced temperature variation (∂T/∂P)Sof liquids in the low‐frequency range from 0.5 Hz to 2 kHz. The relative accuracy of the measurement is of the order of 10−3–10−4. The velocity and absorption coefficient of sound in the low‐frequency range give us information on slow relaxational phenomena. (∂T/∂P)S, which is directly related to the ratio of thermal expansion coefficient to specific heat, can be measured accurately by ac method. The measurement of these quantities provides us with new information on thermodynamic or low‐frequency relaxational behavior of liquids, including critical liquids, liquid crystals near their phase transitions, and biological materials.