Phase relations have been re-examined over the whole compositional range in the system Bi2O3-Y2O3. As a result, four low-temperature stable intermediate phases have been identified : x = 0.215-0.24, x = 0.315-0.35, x = 0.475-0.49 and x = 0.57-0.58 in Bi1-xYxO1.5. Since these phases limit the thermostable region of the delta-Bi2O3 type high-temperature stable phase, the stabilization of the delta-phase with Y2O3 is thermodynamically impossible. Upon heating, the first three phases change smoothly into the delta-phase at about 720 degrees C, 890 degrees C and 1010 degrees C respectively. The last phase dissociates into the delta and Y2O3 phases around 1125 degrees C on heating. Upon cooling, by contrast, the transition rate is extremely sluggish. Because of this sluggishness, the delta-phase can easily be quenched to room temperature in a metastable state.