Phase equilibria in the copper-nickel-selenium system has been investigated using x-ray diffraction and scanning electron microscopy. Ternary isotherms at 703 and 573 K show interesting differences. In the Cu-Se system, Cu 3 Se 2 , CuSe, and CuSe 2 phases decompose at higher temperatures. Only two phases viz., Cu 2-x Se and Cu 2 Se are stable at 703 K. Solid solubility of nickel in these stable phases is ≈2 at %. Tetragonal Ni 3 Se 2 phase has a solid solubility of about 8 at % copper in the ternary system. At 573 K, NiSe phase dissolves ≈5 at % copper. Rapid quenching of alloys on the Cu 2 Se-Ni 3 Se 2 quasi-binary section resulted no change in crystal structure. Phase relations and crystal chemistry of the system has been discussed in detail.