The addition of molten alkali metal salts drastically accelerates the kinetics of CO₂ capture by MgO through the formation of MgCO₃. However, the growth mechanism, the nature of MgCO₃ formation and the exact role of the molten alkali metal salts on the CO2 capture process remains elusive, holding back the development of more effective MgO-based CO₂ sorbents. Here, we unveil the growth mechanism of MgCO₃ under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO₃. The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO₂, a non-crystalline surface carbonate layer of ca. 7 Å thickness forms. In contrast, when MgO(100) is coated with NaNO₃ MgCO₃ crystals nucleate and growth. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO₃. MgCO₃ grows epitaxially with respect to MgO(100) and the lattice mismatch between MgCO₃ and MgO is relaxed through lattice misfit dislocations. Pyramid shaped pits on the surface of MgO, in the proximity and below the MgCO₃ crystals, point to the etching of surface MgO, providing dissolved [Mg²⁺…O^2–] ionic pairs for MgCO₃ growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions.