Phase equilibria of the MgO-Cu2O-CuO system have been determined in air at temperatures ranging from 850 to 1100 degrees C. The starting compositions ranged from 0 to 100% Mg in (Mg1-xCux)O. CuO was quite soluble in MgO. The solubility of CuO in MgO at 850 degrees C was about 9% and at 1050 degrees C was about 20%. The only known ternary Guggenite phase had a composition of Cu2MgO3. The Guggenite phase exists in three forms, namely, Guggenite A (G(A), orthorhombic, space group Pmmn(59)), Guggenite B (G(B), orthorhombic, space group I(0)), and Guggenite X (G(X), monoclinic). Guggenite A was obtained as a single phase in air at x(CuO) = 67% (i.e., 67% CuO and 33% MgO) and 1000 degrees C. It undergoes a phase transition to the Guggenite X phase when it is heated above 1050 degrees C. The Guggenite B phase was obtained in a mixture at x(CuO) = 75% and at temperatures above 1050 degrees C. MgO was not found to be soluble in CuO. For samples with x(CuO) greater than or equal to 67%, the Guggenite A phase coexisted with tenorite (CuO, monoclinic) at temperatures less than or equal to 1000 degrees C and above 1021 degrees C, it was in equilibrium with cuprite (Cu2O, cubic). For CuO compositions between 20% and 67%, a mixture of Guggenite A phase and the periclase phase (MgO, cubic) was obtained at temperatures less than or equal to 1000 degrees C. The decomposition of Cu2MgO3 (Guggenite A) phase was determined at several oxygen partial pressures and the Gibbs free energy of formation of this phase was also calculated.