The use of formamidine sulfinic acid in the textile industry goes back many years, particularly as an agent for the reduction or "vatting" of vat dyes, to form the water-soluble leuco species, when dyeing or printing cellulosic fibers. Many workers have labeled this agent as thiourea dioxide and theorized that its reducing power developed through the existence of two isomeric forms: thiourea dioxide (the sulfone), and formamidine sulfinate. This article has used solid and solution Fourier transform infrared (FT-IR) spectroscopy to show that the name thiourea dioxide is a misnomer since the compound does not exist in the six-valent state, but is stabilized in the four-valent state by formation of a strong internal zwitterion as shown in the article.