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Copper(II) oxide nanopowders exhibit a high catalytic activity in CO oxidation at low temperatures. The combination of in situ XPS, XRD, and HRTEM methods was applied to investigate initial steps of CuO nanoparticles reduction, to identify oxygen and copper species and to revealed structural features in the dependence on reducing power of reaction medium. At the oxygen deficient surface of CuO nanopowders the metastable Cu4O3 oxide was formed under the mild reducing conditions −10–5 mbar CO or CO + O2 mixture with oxygen excess. Destruction of Cu4O3 structures in strong reducing medium (P(CO) ≥ 10–2 mbar) or under UHV conditions resulted in the formation of Cu2O which was epitaxially bounded with initial CuO particle. The reversible bulk reduction of CuO nanopowder to Cu2O at temperatures ∼150 °C can be explained by effortless propagation of Cu2O∥CuO epitaxial front inside the nanoparticle. The model of the surface restructuring along the {−111}CuO → {202}Cu4O3 → {111}Cu2O planes under the reduction of CuO nanopowders is proposed. The initial surface of CuO nanopowders is probably distorted and resembles Cu4O3-like structures that facilitates the CuO x ↔ Cu4O3 transition in mild reducing conditions. Such restructuring results in a unique electronic Cu4O3 structure with high oxygen deficiency and low-valence Cu1+ sites stimulating the formation of highly reactive CO and O2 adsorbed species. It was shown that the most active oxygen species on the surface of CuO x is stabilized as O–, which was previously reported in papers by Roberts and Madix in their study of the copper–oxygen systems.