The phase equilibria of the Co-Cu-Ti system at 850℃ 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℃, no ternary compound is found. The Ti3Cu phase is a metastable phase. The solubilities of Cu in Ti2Co, TiCo, TiCo2 (c) and TiCo3 were determined to be 8.7, 36, 2 and 5.4, respectively. And the solubilities of Co in Ti2Cu, TiCu, Ti3Cu4, and TiCu2 were measured to be 6.6, 0.7, 2.4, and 10.2, respectively. The following 9 three-phase equilibria are observed: (Ti)+Ti2Co+Ti2Cu, Ti2Co+Ti2Cu+TiCo, Ti2Cu+TiCu+TiCo, TiCu+Ti3Cu4+TiCo, Ti3Cu4+TiCu2+TiCo, TiCu2+TiCo+(Cu), TiCo+TiCo2(c)+(Cu), TiCo2(h)+TiCo3+(Cu), and TiCo3+(Co)+(Cu).