The constitution of the plutonium-copper binary alloy as determined by differential thermal analysis is presented. The system is characterized by two congruent melting compounds, PuCu2 (m.p. 865 degrees C.) and Pu4Cu17 (m.p. 954 degrees C.); two incongruent melting compounds, PuCu4 (m.p. 906 degrees C.) and Pu2Cu11 (m.p. 926 degrees C.); three eutectics, 96 atom per cent copper (m.p. 626 degrees), 70.5 atom per cent copper (m.p. 849 degrees C.), and 91 atom per cent copper (m.p. 881 degrees C.); and two peritectics at 75 atom per cent (m.p. 906 degrees C.) and 85.5 atom per cent (m.p. 926 degrees C.). Solid solution was found above 97 atom per cent plutonium. The apparatus, the method of investigation, and the binary alloy phase diagram is discussed.