The experimental method for determining the oxygen content in high Tc superconduc tors is described in detail. This method is applied to determination of oxygen content in high Tc Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O samples in which the stoichiometry is varied by reducing the copper and bismuth concentrations. The oxygen concentration is found to vary linearly with Cu (x= 0-0,2) and Bi (x= 0-0.4) deficiencies in YBa2Cu3(1-x)Oy and Bi2(1-x)Sr2CaCu2Oy respectively. X-ray powder diffraction measurements show that the compound of YBa2Cu3(1-x)Oy, is orthorhombic in the variation range of x=0-0.2