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.