Abstract This study was conducted to investigate the phase behaviour of CO 2 –brine and CO 2 –oil systems under various operating conditions. Through this study, CO 2 solubility measurement tests were carried out for CO 2 –water, CO 2 –brine, and CO 2 –oil mixtures at various equilibrium pressures ranging Peq = 0.7–10.3 MPa and temperatures ranging Texp = 21–40 °C. Additionally, series of oil swelling/extraction tests were conducted at aforementioned experimental conditions using a see-through high- pressure cell to determine the oil swelling factor at various equilibrium conditions. CO 2 solubility measurement tests showed that at constant temperatures, an increase in CO 2 solubility value was observed for CO 2 –water, CO 2 –brine, and CO 2 –oil mixtures when the equilibrium pressure increases. Furthermore, as it was expected for all mixtures, the solubility of CO 2 reduces with increased temperature. In this study, it was also found that at a constant temperature, the oil swelling factor, SF , increases up to a pressure so called extraction pressure, Pext , at which majority of the light to medium hydrocarbon groups in the oil phase are extracted by CO 2 and vaporized into the CO 2 -rich phase. Additionally, it was observed that for the pressures higher than the extraction pressure, the oil swelling factor reduced with equilibrium pressure because more hydrocarbon components were extracted at higher pressures. The extraction pressure was determined at different experimental temperatures and results revealed that the extraction pressure increases by increasing experimental temperature. Comparison of the CO 2 solubility values in oil at extraction pressures corresponding to different experimental temperatures also showed that the major hydrocarbon extraction occurs when a certain amount of CO 2 has dissolved in the oil phase which is called threshold CO 2 solubility, χth . The defined threshold CO 2 solubility was found to be approximately the same for the CO 2 –oil mixture under this study at different temperatures.