Goswami, Parikshit and Basak, Montu (2014) Sulfur Dyes. In: Kirk-Othmer Encyclopedia of Chemical Technology. Wiley. ISBN 9780471238966

Sulfur dyes are synthetic organic substantive dyes, produced by thionation or sulfurization of organic intermediates containing nitro and amino groups. The main characteristic feature of this dye class is that they all contain sulfur linkages within their molecules. They are like vat dyes, which are highly colored, water-insoluble compounds, and they need to be converted into substantive leuco form before application to the textile materials. This conversion needs reducing agents, which sever the sulfur linkage and break down the molecules into water-soluble components (leuco form) that have an affinity toward cellulosic fibers.