A phase diagram is proposed for the system Co oxide/Cu oxide showing the phases which are stable in air at 600-1200 Deg. From x-ray analyses of quenched samples and from high-temp. x-ray analyses, the occurrence of the following phases was established: a cubic spinel phase CuxCo3-xO4 (0 ? x ? 0.3); a cubic rock-salt phase CuyCo1-yO (0 ? y ? 0.39), a phase CuyCo1-yO (0.63 ? y ? 0.68) with a specific structure, a liq. phase, the phase Cu2O, and the phase CuO. The O content of most of the quenched samples consisting of a single phase was slightly higher than that given by the above formulas. The positions of the phase boundaries and the phase transitions were further investigated by D.T.A. and by microscopic observations on quenched samples as well as on samples at high temps. The isomorphism of the compd. Cu2CoO3 (y = 2/3) and the mineral guggenite (Cu2MgO3) is discussed.