The chemical bonding of different {111} Cu–MgO interfaces obtainedby internal oxidation of (Cu, Mg) alloys at T = 900 °C and for an highoxygen activity of a o2 = 10−8, is studied by transmission electronenergy loss spectroscopy (EELS) at high spatial resolution. For polar {111}interfaces (Cu and MgO in topotactical or pseudotwin orientation), it isshown that the terminating lattice plane in magnesia is occupied by oxygenatoms. An important charge transfer is identified at the interface,yielding Cu–L ELNES features corresponding to those of Cu1+ (Cu(I))in its oxide. O–K edge fine structures at the interface are also modified: an edge enlargement and the presence of a low energy shoulder confirm thebonding of oxygen to Cu1+. Consistent with these results, the Mg–Ledge is never modified compared to the MgO bulk phase. Specifically adaptedto the heterophase interfaces, a spatial difference method, based onnormalised spectra (NSD), is applied to estimatethe relative contribution of the ELNES signal in the interface area. In thepresent case of high oxygen activity, the number of copper atoms in theCu(I) oxidized state corresponds to a total occupancy of the outermostmetal plane at the interface.