The dissolution of the surfaces (100), (110), and (111) of MgO in aqueous hydrochloric acid is studied by in-situ AFM experiments in a flow cell with known hydrodynamics,which permits the modeling of the rate of proton transport to the solid surface. Comparison with directly measured rates of dissolution determined via monitoring the absolute height of the surface in real time shows that the dissolution of all three surfaces is a surface-controlled reaction. Examination of the surface morphology shows that the (100) plane dissolves via the growth of etch pits which are of circular or square shape depending on the acid concentration. In contrast, the (110) surface dissolves to form a corrugated surface of parallel ridges whose surfaces are predominantly composed of (100) planes. The (111) surface dissolves via triangular etch pits of a fixed orientation where (100) planes are partly re-expressed during the pit growth. These observation are in concordance with previous reports that the (100) plane is the most stable surface of MgO.