Molecular diffusion has been proved to be the main factor affecting the solubility and miscibility of carbon dioxide (CO2). On the basis of previous work, X-ray computed tomography (CT) technology is developed in this paper to investigate the CO2 diffusion process in brine with varying pressures and a constant temperature, simulating CO2 storage in a saline aquifer. Interface migration and volume expansion in response to CO2 diffusion processes are imaged by use of an X-ray tomographic scanner and quantitatively analyzed. Volume expansion and pressure decline occur simultaneously. Interfacial area and migration velocity were calculated to describe interface migration related to the diffusion process. Combined with Fick's second law, the CO2 diffusion coefficients, which vary with pressure, are calculated in consideration of the mixture volume V-mix and interfacial area A. The CO2 diffusion process is divided into two stages, early and late. Two effective diffusion coefficients, corresponding to these two stages, were calculated. The results representing the early stages are in the range (1.0-2.2) x 10(-7) m(2).s(-1), which is 2 orders of magnitude larger than those for late stages of the experiments, in the range (1.9-2.1) x 10(-9) m(2).s(-1). The first coefficient increases linearly as the pressure increases from 2 to 8 MPa, showing strong sensitivity to pressure. In contrast, diffusion coefficients during the late stage can be assumed to be constant for a wide range of pressures. The maximum velocity of the interface migration tested here can reach 1.5 x 10(-5) m.h(-1). Equilibrium solubility is in the range of 0.20-1.24 mol.L-1. This is of great significance for understanding the mass-transfer process in CO2 storage.