The phase equilibria of the Co–Cu –Ti system at 850 °C are investigated using a high-efficiency diffusion couple and six key ternary alloys. The samples are examined by means of optical microscopy, scanning electron microscopy, and electron probe microanalysis. At 850 °C, no ternary compound is found. The Ti 3 Cu phase is a metastable phase. The solubilities of Cu in Ti 2 Co, TiCo, TiCo 2 (c) and TiCo 3 were determined to be 8.7, 35.8, 2.1, and 5.4 at.%, respectively. And the solubilities of Co in Ti 2 Cu, TiCu, Ti 3 Cu 4 , and TiCu 2 were measured to be 6.6, 0.7, 2.4, and 10.2 at.%, respectively. The following 9 three-phase equilibria are observed: (Ti) + Ti 2 Co + Ti 2 Cu, Ti 2 Co + Ti 2 Cu + TiCo, Ti 2 Cu + TiCu + TiCo, TiCu + Ti 3 Cu 4 + TiCo, Ti 3 Cu 4 + TiCu 2 + TiCo, TiCu 2 + TiCo + (Cu), TiCo + TiCo 2 (c) + (Cu), TiCo 2 (h) + TiCo 3 + (Cu), and TiCo 3 + (Co) + (Cu).