Phase equilibria in the Ag-CuO-Cu2O system were experimentally determined using thermal analysis, and structural and compositional studies. Three reactions were observed in air: (1) L1 = CuO + Ag, (2) L2 = CuO + L1, and (3) Cu2O = CuO + L2. The evolution and absorption of oxygen accompanied these reactions. At oxygen partial pressures below 0.02 bar, the reactions L1 = Cu2O + Ag and L2 = Cu2O + L1 were found. Based on isobaric projections in the Ag-CuO-Cu2O system, two invariant reactions, L1 = CuO + Cu2O + Ag and L2 = CuO + Cu2O + L1, were deduced.