[en] Stable and metastable phase relationships in the system ZrO₂-ErO_1.5 were investigated using homogeneous samples prepared by rapid quenching of melts and by arc melting. The rapidly quenched samples were annealed in air for 48 h at 1680 degrees C or for 8 months at 1315 degrees C. Two tetragonal phases (t- and t'-phases) were observed after quenching samples heated at 1690 degrees C to a room temperature, whereas one t-phase and cubic (c-) phase were found in those treated at 1315 degrees C. Since the t'- phase is obtained through a diffusionless transformation during cooling from a high-temperature c-phase, t- and c- phases can coexist at high temperature. The t- and c-phases field spans from 4 to 10 mol% ErO_1.5 at 1690 degrees C and from 3 to 15 mol% ErO_1.5 at 1315 degrees C. The equilibrium temperature T₀^t-m between the t- and monoclinic (m-) phases estimated from A_s and M_s temperatures decreased with increasing ErO_1.5 contents