In this paper, the mass transfer of CO2 into a reservoir brine sample is studied experimentally at high pressures and elevated temperatures. The equilibrium concentration of CO2 in the reservoir brine and the density of CO2-saturated brine are measured by saturating the brine with CO2. The mass-transfer rate of CO2 into the brine is determined by monitoring the pressure decay inside a closed, visual, high-pressure PVT cell. It is found that the density of the brine with dissolved CO2 increases linearly with CO2 concentration. As CO2 gradually dissolves into the brine by molecular diffusion, a concentration-induced density gradient is generated near the CO2-brine interface. Under the influence of gravity, this concentration- induced density gradient causes natural convection, which accelerates the mass-transfer rate of CO2 into the brine. The modified diffusion equation with an effective diffusivity is applied to model the mass-transfer process. It is found that the determined effective diffusivities of CO2 in the reservoir brine are almost two orders of magnitude larger than the molecular diffusivities of CO2 in water or similar reservoir brines. The detailed experimental results show that the density-driven natural convection greatly accelerates the dissolution process of CO2 in brine. This means that loss of CO2 in brine can be significant in an enhanced oil recovery operation using CO2 flooding in an oil reservoir with a bottom water aquifer. More importantly, the accelerated mass transfer due to the density-driven natural convection significantly increases the geological sequestration rate of CO2 in deep saline formations.