A process for the purification of diaminophenothiazinium compounds and particularly of methylene blue is described. The process provides for simple and affective purification by reduction of the post synthesis or commercially available diaminophenothiazinium compound to form a reduced complex thereof. This can then be purified in a more straightforward mann&r than the original compound by for example recrystaliisation before being allowed to oxidise back to the diaminophenothiazinium compound.
