The anionic composition, structural parameters, optical properties and reduction behavior of Cu-MgO solid solution in hydrogen dramatically change after exposure to air. The air-exposed Cu-Mg oxide contains a lot of CO 3 2- and OH- anions. Its reduction proceeds via two stages: (1) diffusion of Cu2+ to the surface and (2) chemical interaction of Cu2+ with hydrogen. The effective activation energy gradually increases from that of the chemical step (65 kJ/mol) to that of the transport step of Cu2+ diffusion (130 kJ/mol). This behavior follows the "compensation effect", which is close to those reported earlier for CuO reduction. On the contrary, reduction of Cu2+ from the Cu-Mg oxide sample, which was not exposed to air after thermal pretreatment in the inert gas, proceeds in one step at 120-160°C with the effective activation energy of 19 kJ/mol, which is manifold less than the reported effective activation energies for various Cu-oxide systems. Water molecules eliminate from the sample slowly along with further heating up to 450°C.@en