Summary: A 1 M solution of potassium permanganate was sealed in a gold capsule and was treated under conditions of temperature and pressure up to 800 °C and 2·5 kb. Different kinds of products were produced under different conditions of synthesis, and potassium manganate, birnessite, cryptomelane, hausmannite, manganosite, and a new phase appeared. The chemical composition of the new phase was determined as K₂Mn₄O₉ and its X-ray powder diffraction pattern was indexed on a hexagonal cell with a 11·295 and c 21·870 Å.