The chemical bonding of different {111} Cu–MgO interfaces obtained by internal oxidation of (Cu, Mg) alloys at T = 900 °C and for an high oxygen activity of ao2 = 10−8, is studied by transmission electron energy loss spectroscopy (EELS) at high spatial resolution. For polar {111} interfaces (Cu and MgO in topotactical or pseudotwin orientation), it is shown that the terminating lattice plane in magnesia is occupied by oxygen atoms. 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 the bonding of oxygen to Cu1+. Consistent with these results, the Mg–L edge is never modified compared to the MgO bulk phase. Specifically adapted to the heterophase interfaces, a spatial difference method, based on normalised spectra (NSD), is applied to estimate the relative contribution of the ELNES signal in the interface area. In the present case of high oxygen activity, the number of copper atoms in the Cu(I) oxidized state corresponds to a total occupancy of the outermost metal plane at the interface.