The chemical mode of action as a flame retardant of calcium carbonate nanoparticles combined with ammonium polyphosphate in polypropylene was investigated. Reduction in burning rates for 0.5 mm thick samples were observed without appreciable char formation up to 30 wt % loading of additives. Thermogravimetric analysis (TGA) of the mixture of CaCO(3) and ammonium polyphosphate (APP) showed that calcium carbonate nartoparticles react with ammonium polyphosphate before the degradation of the phosphate chains. TGA-FTIR studies of the polymer composite samples and powder mixtures of the additives confirmed the evolution of ammonia and carbon dioxide due to interaction between the additives. X-ray diffraction (XRD) analysis of chars, obtained after burning the films, showed definite diffraction peaks corresponding to that of calcium metaphosphate. The inert gasses produced by the interaction of the additives hindered the advancing flame and, thus, reduces the burning rates, at times even without char formation. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1866-1873, 2011