Abstract The stoichiometry and the morphology of MgO adlayers epitaxially grown on Ag(100) are studied by means of X-ray photoelectron spectroscopy (XPS) and high-resolution spot profile analysis low-energy electron diffraction (SPA-LEED). For comparison, Mg deposited at 200 K is oxidized after and during growth, respectively. The post-oxidized film is dominated by stoichiometric MgO but shows also non-stoichiometric species. Annealing the film to 700 K reduces the latter components. Strong differences have been found for the epitaxy of both kinds of MgO films, depending on the preparation condition. The post-oxidized film shows a clear diffraction pattern with 12-fold symmetry due to MgO(100) grains in three rotational orientations only after annealing to 700 K. Depositing and oxidizing simultaneously the Mg the oxide film shows a better film quality (no metallic Mg, a small MgO 2 fraction) and epitaxy with 1×1 already at 200 K.