It is well know that the degree of oxidation for the most superconducting cuprate systems play a very important role in the properties of superconductivity. Thus, the precise determination of oxygen content and nonstoichiometry had been actively perfromed in the time when the high-Tc cuprate superconductors wer discovered. In most cased the oxygen contents of cuprate superconductors were determined by the thermogravimetric technique and the iodometric titration. In the present study the maximum oxygen contents of he superconducting oxides, $YBa_2Cu_3O_{7-x}$ have been determined by the thermogravimetric analysis (TGA). Decomposition reactions in hydrogen atmosphere wer also determined by the x-ray diffraction analysis, especially by consdiering the several inflections echibited in the TGA curve of $YBa_2Cu_3O_{7-x}$ in hydrogen reduction to $950^{circ}C$. Particular decomposition reaction at the long plateau of $615-760^{circ}C$ has been identified as chemical reactions, $Ba_4Y_2O_7; ightarrow;Ba_2Y_2O_5$ + 2BaP and $3BaCuO_2; ightarrow;Ba_2Cu_3O_5;+;BaO_{x}$ without weight loss.