Thionine, also known as Lauth's violet, is the salt of a heterocyclic compound.

The dye is structurally related to methylene blue, which also features a phenothiazine core.

[1] The dye's name is frequently misspelled with omission of the e, and is not to be confused with the plant protein thionin.

[4] Thionine can also be used in place of Schiff reagent in quantitative Feulgen staining of DNA.

When methylene blue is "polychromed" by ripening (oxidized in solution or metabolized by fungal contamination,[6] as originally noted in the thesis of Dr D L Romanowski in 1890s), it forms thionine and all the Azure intermediates.