Amlndiminomethanesulflnic acid, formamidinesulfinlc acid, or thiourea dioxidé Is the compound made by the reaction of thiburea arid hydrogen peroxide. This compound Is rémárkable In that it readily reduces cadmium to the metal from aqueous solutions. Zinc metal does not precipitate, Dithionite, S2042-, occurs in alkaline solutions of aminoiminomethanesulfinic acid. Soliitions of dithionite do not reduce cadmium to the metallic state and finding carbon in the precipitated cadmium Indicated that the specieé responsible for the strong reducing properties of the compound is a radical ion formed by a homolytic cleavage of the carbon-sulfur bond. A scheme to explain the above observations is presented.