For the application of supercritical carbon dioxide as a medium for polymer processing, it is important to have reliable thermodynamic data of the systems involved. In this study, experimental cloud-point data of binary systems of ethylene-poly(ethylene-co-propylene) (PEP) are presented for different molecular weights, 8.7 and 51 kg/mol, respectively. Also, the influence of carbon dioxide on the binary system ethylene-PEP51 is studied. Additionally, the experimental results have been modeled using the statistical associating fluid theory equation of state. Using a single isopleth to determine the binary interaction parameter between PEP and CO2, intermediate compositions can adequately be predicted.