The pressure-volume-temperature (PVT) properties of polymer/gas Solutions are characterized by determining their volume dilation at a known temperature and pressure. This paper presents the results of an experiment that measured the volume swelling of linear and branched polypropylene (PP) with CO2 using a new PVT measurement apparatus. The swellings and specific volumes for linear and branched PP/CO2 are compared. The swelling mechanism that Occurred due to CO2 dissolution, and the effects of the branched structure on Volume swelling are investigated and discussed. Due to its entangled branch structure, the branched PP underwent a lesser degree Of Volume swelling than did the linear PP.