When carbon dioxide (CO 2) is layered on top of brine, the diffusion occurring at the interface generates an intermediate layer denser than the brine one. This may generate a convective process that accelerates CO 2 dissolution into the brine. Here, we use the shadowgraph method and a sample cell geometry with a large width/height aspect ratio to investigate the 3D-behaviour of the convective dissolution of CO 2 in brine in a free medium. This process is driven by the instability of the denser boundary layer formed at the interface between the two phases and is totally independent of the overall vertical size. When the purely diffusive mass transfer get disrupted, we observed the development of complex 3D structures, consisting of CO 2-rich falling sheets at the liquidliquid interface. This pattern could not be detected in 2D Hele-Shaw studies, mainly used in the literature. The experiments have been performed over a broad range of CO 2 pressures, close to the process-relevant conditions, and show that the convective mass transfer is governed by the critical thickness of the mixing layer at the onset of convection, and that the process can be described by a scale-free universal model.