The kinetics of synthesis was studied in situ on an x-ray diffractometer by the time and temperature dependences of concentration of the formed YBa2Cu3O7-y (YBCO) phase. The largest synthesis rate is achieved at 750-780 degreesC. The effective activation energy is minimum at 700 degreesC: E-a = 20-30 kJ mol(-1). In the synthesized YBCO, the parameters of structure inhomogeneity and the sizes of coherent scattering regions were determined.