The formation of Y-Ba-Cu-O phases, including the high Tc superconducting YBa2Cu3O7−x (Y123) phase, during solid state reaction of the mixture of Y2O3: BacO3: CuO, in molar ratio of 0.2:0.6:1 to conform the formula of the perovskite (Y0.4 Ba0.6)CuO3 composition was studied by means of DTA, DTG and X-ray difractometry(XRD). For continuous heating at 10 C/min, it was found that the Y123 phase exists at 790–995 C, and gradually disappears at higher temperatures. The insulating Y2BaCuO5 (Y211) phase exists at 985 to 1190 C, the maximum experiment temperature. While the semiconducting YBa3Cu2O7-y, (Y132) phase coexists with the Y123 phase. For isothermal heating, it was found that the most appropriate temperature and time for the formation of the Y123 phase is 900 C and 16 hours. Semiquantitative data on the amount of phases presented, including the above mentioned phases and other binary Ba-Y-O or Ba-Cu-O phases, during heating are also given. High temperature superconductor with a Tc of around 93 K can be synthesized reproducibly by the processes based on this study.