The synthesis of YBa2Cu3O7-x from Y2O3, BaO2 and CuO has been studied in the temperature range 330-930 degrees C in order to identify the intermediate phases which appear prior to the formation of the Y-123 compound. The first reaction between BaO2 and CuO in the mixture begins at approximate to 330 degrees C. Barium cuprates of composition BaCuO2+x(0 less than or equal to x less than or equal to 0.5) are formed, with BaCuO2 as the most stable phase. BaO2 is mainly consumed in these reactions up to approximate to 750 degrees C. Barium cuprates and Y2O3 react finally above 750 degrees C, giving YBa2Cu3O7-x.