The effect of combination between a trace of halogenated compounds (such as ferric chloride and ammonium bromide) and Ni2O3 particles on the carbonization of polypropylene (PP) was investigated during combustion. The results showed a synergistic catalysis of combined halogenated compounds with Ni2O3 in promoting the formation of the residual char during combustion. The investigation on the promotion mechanism showed that halide radical releasing from halogen-containing additives worked as a catalyst to accelerate dehydrogenation-aromatization of degradation products of PR which promote the degradation products to form the residual char catalyzed by nickel catalyst. The results of GC and GC/MS confirmed that halogenated compounds could accelerate dehydrogenization and aromatization of degradation products of PP. The morphological observation showed that longer fiber-like Structure was formed in the residual char of PP/halogenated compound/Ni2O3 composites comparing with PP/Ni2O3 composite. As a result, the presence of a trace amount of halogenated compounds could significantly improve the flame retardancy of PP/Ni2O3 composites, which is proved by cone calorimeter tests. (C) 2009 Elsevier Ltd. All rights reserved.