The equilibrium phase diagram of the plutonium-cerium-copper system is presented, based on data obtained by differential thermal analysis, metallography and electron-microprobe x-ray analysis. Liquid-solid phase transitions were found between 419 and 1083°C. Ten crystalline phases solidify as primary phases from molten plutonium-cerium-copper solutions. No ternary compounds occur in the system. The ternary system is divided into two subsystems by the quasibinary section, CeCu2-PuCu2. Continuous solid solubility occurs along this join with neither a maximum nor a minimum. Solid solubility also occurs between the congruent compound, CeCu6, and the incongruent compound, Pu2Cu11. The ternary system is characterized by a single eutectic (4at.%Pu-68at.%Ce-28at.%Cu at 419°C), four peritectics, and two inversion points.