Process for preparing diaminophenothiazinium type compounds having a step for purification of derivatives (II). The diaminophenothiazinium type compounds having the following formula (I): in which R1, R2, R3, R4, R5, R6, R7, R8, R9 and R10 may be chosen, independently of the others, from the group constituted of: a hydrogen atom, saturated or unsaturated, linear or branched C1-C6 alkyl groups, or C3-C6 cycloalkyl groups, optionally substituted with one or more functions chosen from a halogen atom, and a C1-C6 alkoxy, C1-C6 alkoxycarbonyl or —CONH2 function, aryl groups optionally substituted with one or more functions chosen from: a C1-C4 alkyl, a halogen atom, and a C1-C6 alkoxy, C1-C6 alkyloxycarbonyl or —CONH2 function, in addition, each of R5, R6, R7, R8, R9 and R10 may be chosen, independently of the others, from the halogen atoms: F, Cl, Br and I, and X− represents an organic or inorganic anion.