Phase relations were determined in the Cu-Bi-Se phase system at 300, 350, 400, 450, 550, 650 and 750degreesC. Four ternary phases, phase A-D, have been synthesized. Phase A (Cu4Bi4Se9) is isotype with Cu4Bi4S9, and phase B (Cu1.7Bi4.7 Se-8) and phase C (Cu3Bi5Se9) have the same structure type as P-3 and P-4 respectively of the pavonite homologous series. Phase D (Cu3Bi2Se6) was present in insufficient amount for x-ray-crystallographic studies. Apparently no sulphide corresponding to phase D exists. Phase A is stable up to 447degreesC. Phase B is stable at all temperature levels up to 550degreesC. Phase C has only been synthesized at 450degreesC. At 440degreesC it is not present. Phase D is stable at 300 and 350degreesC. Phase A displays no detectable variance in chemical composition. Maximum range in chemical composition of phase B was recorded at 450degreesC. At this temperature the compositional field of the phase is triangular in shape, defined by the compositions Cu8.8Bi34.7Se56.5, Cu12.3Bi32.0Se55.7 and Cu13.4Bi31.9Se54.7. Phase C varies in composition between Cu16.2Bi30.3Se53.5 and Cu14.8Bi31.1Se54.1 and phase D at 350degreesC between Cu28.8Bi17.1Se54.1 and Cu26.0Bi18.7Se55.3. On the Cu-Se join Cu, CuSe, Cu2Se and Se dissolve negligible amounts of Bi, whereas substantial amounts of this metal may dissolve in Cu2-xSe. At 450degreesC it may reach the composition Cu0.99Bi0.35Se1.00. On the Bi-Se join two intermediate phases are present, Bi2Se3 and BixSey. The former has near stoichiometric composition, dissolving max 0.8 wt.% Cu. BixSey displays a considerable variance in chemical composition and may dissolve up to 1 wt.% Cu. With increasing temperature a liquid field projects from the Bi corner into the phase diagram. At 400degreesC a small liquid field has developed around Cu35Bi10Se55. At 550degreesC the two liquid fields present at 400 and 450degreesC have merged into one field stretching from the bismuth corner of the phase diagram onto the Cu-Se join.