AbstractPhase formation under high pressure in the chalcopyrite–delafossite system has been studied at 5 GPa and 500–1000 °C. The parent chalcopyrite composition used in this study was slightly Cu-deficient, corresponding to the chemical formula (Cu,Ga)0.905GaS2. It was revealed that under high-pressure and high-temperature conditions, a mixture of the parent phases partly decomposes to form Cu2S and Ga2O3. The defect chalcopyrite (Cu,Ga)0.905GaS2 splits into a near-stoichiometric CuGaS2 and a Cu-deficient cubic (sphalerite-like) phase of the (Cu,Ga)0.8S composition. Such a phenomenon has been considered in terms of characteristic features of chalcopyrite and sphalerite crystal structures. Delafossite CuGaO2 at 5 GPa and 1000 °C was found to recrystallize entirely, which results in the disappearance of the stacking faults in its layered structure.