A process of in situ forming a dye discharge reagent on at least a portion of surface of a dyed substrate, effected by applying two or more compositions comprising reactants on the substrate, such that the reactants undergo a chemical reaction on the substrate to form the reagent is provided herein, as well as a method for dye discharging a dyed substrate using the process of in situ forming a dye discharge reagent, and a dye discharge composition comprising said dye discharge reagent. In some embodiments of the invention, the reactants are thiourea and a peroxide, and the dye discharge reagent is formamidine sulfinic acid (FSA).