Ethylene-propylene diene rubber (EPDM) and isotactic polypropylene (iPP) blends have widest industrial applications that require a degree of flame retardancy. Halogen-free intumescent technology based on phosphorous salt is a significantly advanced approach to make the polymer flame-retardant. Both ammonium polyphosphate and ethylene-diamine phosphate are important intumescent compounds. Their combination with carbonific and spumific agents were studied in binary blends of EPDM/PP. The polymer system was vulcanized online during melt mixing. Intumescent flame-retardant polymer systems exhibit good flame-retardancy with optimum comparable physiomechanical, electrical, and fluid resistance properties, including lower smoke emission, which is essential to protect people because the visibility remains unaffected in the event of fire. Pronounced charring and intumescent effect appear to enhance the flame-retardancy of the polymers. Possible expected intumescent mechanism is proposed based on the nonpyrolysis mechanism for the flame-retarded polymer and the intumescent components. (C) 2004 Wiley Periodicals, Inc.