It is shown that in the high-TcYBa2Cu3O∼7superconductor the critical temperature is a function of the orthorhombic distortion (b–a)/aof the unit cell. From the extrapolation of the (b - a)/aratio againstTc, a maximum critical temperature of 66 K for the tetragonal phase of YBa2Cu3O∼7was predicted. From the correlation between the transition width δTcand the orthorhombic distortion there was deduced an upper limit forTc, in the orthorhombic phase of YBa2Cu3O∼7of 94·5K.