In this paper, an experimental technique is developed to study the interfacial tension phenomenon and visual interactions of crude oil-brine-CO 2 systems at different pressures and temperatures. The major component of this experimental set-up is a see-through windowed high-pressure cell. By using the axisymmetric drop shape analysis (ADSA) for the pendant drop case, this new technique makes it possible to determine the interfacial tension (IFT) and to visualize the interfacial interactions among crude oil, brine and CO 2 under practical reservoir conditions. More specifically, IFT of the crude oil-brine-CO 2 system is measured as a function of pressure and temperature, respectively. For the crude oil-CO 2 system, it is found that the dynamic IFT gradually reduces to a constant value, i.e., the equilibrium IFT. Meanwhile, a number of important physical phenomena are observed after the crude oil is made in contact with CO 2 . In particular, the oil swelling effect, light-ends extraction, initial turbulent mixing and wettability alteration are the major characteristics of the CO 2 flooding processes. There always exists a constant low IFT (i.e., partial miscibility) as long as the pressure is higher than a threshold value. No ultra low or zero IFT between the crude oil and CO 2 is found, regardless of the operating pressures and temperatures tested in this study. For the crude oil-brine-CO 2 systems, wettability between crude oil and needle surrounded by CO 2 -saturated brine phase is different from that of the crude oil-CO 2 systems. In addition, immiscibility between CO 2 -saturated crude oil and CO 2 -saturated brine is still observed at P=28.196 MPa and T= 58°C. Therefore, this laboratory study shows that partial miscibility between the crude oil and CO 2 occurs in the reservoirs and that wettability alteration may considerably improve the oil recovery in a water-wet reservoir during CO 2 flooding processes.