Free solutal convection refers to the mixing process induced and sustained by local density differences arising from solute dissolution. This process underpins the long-term storage of carbon dioxide (CO2) following its injection and dissolution in the formation brine of subsurface rock formations, such as saline aquifers. Direct experimental evidence of free solutal convection in porous rocks is to-date still lacking, leaving large uncertainties on the realized rate of CO2 dissolution and its contribution toward storage. Using an analogue solute–solvent pair and 4D X-ray computed tomography, we report direct observations of this mixing process in rock core samples, including sandstones and carbonates. The imagery is used to characterize the mixing structures that arise upon solute dissolution and to quantify differences between the rock types. Thus, we compute the temporal evolution of spatial moments of the concentration distribution to derive practical properties, such as the effective transport velocity of the solute plumes. Unlike previous studies on random bead packs, we observe that these measures do not scale well with core-scale rock properties (permeability, porosity, Rayleigh number) and are influenced by microscale rock characteristics (subcore and pore-scale heterogeneities). The latter may need consideration when evaluating the CO2 storage potential of candidate formations.