This study presents an innovative dilatometer that can accurately measure the pressure-volume-temperature (PVT) properties of polymer/CO₂ solutions. The density (or equivalently, the specific volume) of a polymer/CO₂ solution is determined by measuring the mass and volume flow rates of the solution. A positive-displacement gear pump mounted on an extruder is used to measure the volume flow rate of the solution. The positive-displacement volume of the gear pump was determined in calibration experiments with water and oil with the aid of a syringe pump. The gear pump was mounted on a tandem extrusion line. The temperature of melt was precisely controlled and homogenized by using the second extruder and a heat exchanger, which included a static mixer. The pressure was controlled by the rotational speed of the screw in the second extruder. In order to reduce leakage across the gear pump, the difference between the upstream and downstream pressures was minimized using a variable resistance die attached to the downstream outlet of the gear pump. A critical set of experiments was carried out to measure the specific volumes of various pure polymers. The measured results were compatible with the known PVT data, confirming the validity of the system. The experirnental data were modeled using the equation-of-state (EOS) to predict the relationships above the experimental ranges. Finaily, the system was used to rneasure the specific volumes of various polyrner/CO₂ solutions. A rnetered amount of gas was injected into the polymer melt Stream in the first extruder and the injected gas was cornpletely dissolved in the melt in the second extruder of the tandem foam extrusion Iine. The forrned single-phase polymer/C0₂ solution was forwarded to the gear pump, and its specific volume was measured by comparing the volume and mas flow rates. The sweiling of the polymer melt due to the dissolved CO₂ was characterized as a function of CO₂ concentration, temperature, and pressure. Efforts were made to rnodel the experirnental results.