The surface morphology of two types of high specific surface area MgO powders (an ex-hydroxide MgO and a smoke one, eroded by H2O and CO2 attack) was studied by high resolution transmission electron microscopy (HRTEM) and infrared spectroscopy of adsorbed probe molecules. HRTEM images show that significant morphological defects are present on the surface of MgO microcrystals in both cases. Insights at a molecular level provided by the IR spectra of adsorbed probe molecules (CO, CD3 CN, D-2) indicate that Mg2+ ions on corners, edges and (1 0 0) faces are exposed in comparable amount on the surface of the microcrystals of both kind of powders. By contrast, relevant differences were found for multiple cationic sites and reactive Mg2+O2- pairs located in region with enhanced irregularity, constituted by steep successions of steps of one unit-cell dimension. This suggests that, besides the presence of different types of single morphological defects, the presence of ensembles of such sites must be taken into account when adsorption properties and surface reactivity of different type of microcrystalline MgO materials are compared.