A process for the purification of diaminophenothiazinium compounds, and particularly of methylene blue, is described. The process provides for simple and effective 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 manner than the original compound by, for example, recrystallization before being allowed to oxidise back to the diaminophenothiazinium compound.
