PURPOSE: To easily obtain a germanium-platinum compd. in low-temp. conditions by allowing germanium to react with platinum in a supercritical or subcritical high-temp. and high-pressure water.

CONSTITUTION: The specified quantity of water 5 and preferably powdery Ge or Ge oxide 6 placed in a pressure vessel 1, and a platinum sheet 4 is placed on a support 3. Acid or alkali can be added to increase the solubility of Ge. A lid for the vessel 1 is closed, and the vessel 1 is heated to a specified temp. by a heater 2. The pressure in the vessel 1 is produced by steam and determined by the amt. of water. Heating is continued until the water is made supercritical or subcritical to dissolve Ge, and Ge is slightly dissolved in water. The dissolved Ge reacts with platinum to form a Ge-platinum compd. on the platinum sheet 4. In this case, compds. having different Ge-to-platinum ratios are obtained depending on the used Ge compd. and the pressure and temp. of water, and the thickness of the compd. film is determined by the reaction time.