The isothermal Cu-Ti-Zr phase diagram at 800 C was constructed by means of 36 equilibrated alloys. Electron microprobe analyses were used to determine the phase compositions and phase relationships. Most of the Cu-Zr binary intermetallic phases show a large Ti solubility and extend to the ternary region. CuTi 2 and CuZr 2 form a complete solid solution as a Cu(Ti,Zr) 2 phase. One ternary phase, Cu 2 TiZr, formed at the central region. These phase relationships are quite different from the 703 C isotherm mainly determined by X-ray analysis.