THREE DIHYDRATE FORMS B, C AND D AND A MONOHYDRATE FORM E OF METHYLTHIONINIUM CHLORIDE ARE DESCRIBED. FORMS B, C, D AND E CAN BE PREPARED UNDER CONTROLLED HUMIDITY AND TEMPERATURE FROM METHYLTHIONINIUM CHLORIDE WITH HIGHER WATER CONTENT OR CONVERSION OF A HYDRATE. THE HYDRATES CAN BE INCORPORATED IN PHARMACEUTICAL COMPOSITIONS. FIGURE 1