Significance The grand challenge of reducing CO 2 emissions requires the development of cost-effective CO 2 sorbents. Based on the theoretically obtainable weight-normalized CO 2 uptake, MgO-based materials promoted with molten salts are attractive sorbents when compared to amines or metal organic frameworks. However, there is very little understanding of the processes that occur at the atomic-to-micro scale during CO 2 capture conditions, hampering the advancement of such sorbents. Combining X-ray and electron-based characterization techniques, we observe that MgCO 3 crystals form via nucleation and growth at the interface between MgO and the molten salt and are oriented with respect to the MgO(100) surface. Hence, more-effective MgO-based sorbents will require maximizing the interfacial area and the number of nucleation sites at the interface.