For the first time, analytical methods are used to define and investigate the properties of oxygen-created internal (chemical) pressure Pi in HTS YBa2Cu3O7−y and Tl2Ba2CuO6+x samples. We predict values for the critical temperature Tc, values of dTc/dPi for the main crystallographic axes, and also the possibility of synthesizing the compounds YBa2Cu3O7−y and Tl2Ba2CuO6+x in which Tc is equal to 166 and 92.3 K, respectively.