The present invention relates to : a process for preparing 3, 7-bis-(dimethylamino)-phenothiazin-5-ium bromide or chloride; a method of converting 3, 7-bis-(dimethylamino)-phenothiazin-5-ium bromide to 3, 7-bis-(di-methylamino)-phenothiazin-5-ium chloride; and the purification of 7-bis-(di-methylamino)-phenothiazin-5-ium chloride by crystallization from aqueous solution of hydrochloric acid, leading to a pharmaceutically acceptable 3, 7-bis-(di-methylamino)-phenothiazin-5-ium chloride (methylthioninium chloride, methylene blue, MTC) of formula I below reported.