Abstract: The mass transfer by diffusion of CO₂ molecules between oil and aqueous phases is of great significance to the enhanced oil recovery by CO₂ flooding in tight reservoirs. Through combining experimental measurement data with theoretical diffusion model, this paper simultaneously determines the diffusion coefficients of CO₂ molecules in oil and aqueous phases in the carbonated water-crude oil binary system using the trial-and-error method. This study focuses on the influence of initial pressure of the oil-water mixing system on the diffusion coefficient, and deeply explores the variations of the density of oil-aqueous phases, CO₂ concentration, oil-water interface movement, and CO₂ diffusion front position in the oil phase during diffusion. The results show that:(1)As CO₂ diffuses from the aqueous phase into the oil phase, the pressure of the carbonated water-crude oil system increases. After 10 hours of diffusion at 60℃, the pressure is increased by 24% to 31% for the initial pressures of 15.39,19.25 and 22.82 MPa (Experiments 1 to 3). (2)Near the two-phase interface, the density of the aqueous phase and CO₂ concentration in the aqueous phase gradually decrease, while the density of the oil phase and CO₂ concentration in the oil phase gradually increases, causing the volume of the oil phase to expand and that of the aqueous phase to shrink, and the two-phase interface moves toward the aqueous phase. (3)The higher the initial pressure, the greater the diffusion coefficient. The CO₂ diffusion coefficient in the aqueous and oil phase in experiments 1 to 3 is increased by 52% and 9.2%, respectively, indicating that the CO₂ diffusion coefficient in the aqueous phase is more sensitive to the initial pressure. (4)The higher the initial pressure, the farther the CO₂ diffusion front migrates in the oil phase within the same time period. After 100 hours of diffusion, the migrating distances of the front positions in experiment 1 to 3 are 4.11 cm, 4.32 cm and 4.43 cm, respectively.