Two isothermal sections of the Co–Cu–Ti ternary system at 973 and 1123 K were experimentally determined using the diffusion triple technique together with scanning electron microscopy and electron probe microanalysis. The solubility of Cu (substituting Co) in CoTi increased from 22.8 at.% at 973 K to 28.1 at.% at 1123 K while that of Co (substituting Cu) in CuTi decreased from 11.1 to 8.8 at.% accordingly. In addition, the solubility limits of the third element in the binary compounds CoTi2, CuTi2, Cu4Ti3, and Cu3Ti2 were remarkable. Besides the solubility change, we found the Cu2Ti phase was stable at 1123 K but disappeared at 973 K. A ternary compound “m” with a composition range covering Co10Cu57Ti33 was detected at both isothermal sections. An invariant reaction Cu4Ti3 + CoTi ↔ CuTi + m at a temperature between 973 and 1023 K was deduced. Further investigations are necessary to confirm the reactions among Cu, Cu4Ti, Cu2Ti, Cu3Ti2, and “m” between 1023 and 1123 K.