An intumescent flame retardant (IFR), pentaerytheritol phosphate ester melamine salt (PPEM) was synthesized and chemically modified by magnesium oxide (MgO) or/and calcium oxide (CaO), respectively. The fire retardant efficiency of MgO and CaO as synergistic agents in polypropylene (PP) was evaluated by normal fire testing methods. The results showed that MgO and CaO exhibited a synergistic effect on fire retardancy of PP. The addition of 0.2% of MgO or CaO into PP/PPEM composite could decrease the self-extinguishing time and improve the limited oxygen index (LOI) obviously. The cone calorimeter (CONE) tests and the thermal analysis revealed that the introduction of MgO and CaO neutralized the acidity of PPEM, which leads to a thermal stabilization and a delay of the intumescent process of the system, and promote the formation of a high quality charred layer which can prevent PP from decomposing. Thus, a suitable amount of MgO and CaO acts as a synergistic agent in the flame-retarded PP/PPEM system. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012