Effects of the heating rate (100−6000 °C/h) to a peritectic temperature (Tp – 1015 °C) on conversion of Y2O3−BaCuO2−CuO and Y2BaCuO5−BaCuO2−CuO precursor powders into YBa2Cu3O7−y (Y123) were studied. Both precursor powders were rapidly converted into the Y123 phase. A volume fraction of the Y123 phase was more than 50%, even at a relatively fast heating rate of 3000 °C/h. At about 100−200 °C/h, which correspond to the rates of the first heating cycles of practical melt processes, almost 100% of the precursor powders were converted into a Y123 phase. Microstructures were studied in respect to Y2BaCuO5 (Y211) particle size, distribution, gas evolution, and nucleation mechanism of Y211 particles.