Abstract: A flame retardant prepolymer composition for imparting flame retardance to cotton, cellulosic textiles, and certain other materials; the process for preparing the prepolymer composition; and the application technique of said prepolymer composition are disclosed. Tetrakis(hydroxymethyl)phosphonium sulfate, a THP salt, is acidified, ammonia gas is bubbled into the mixture to achieve a constant boiling, these steps are repeated a number of times, and the prepolymer mixture thus prepared is then applied to a substrate, followed by treatment with a caustic solution.