This study was aimed to investigate the influence of calcium carbonate (CaCO3), a widely used filler, on the fire retardancy of intumescent polypropylene composites. Two intumescent systems based on (1) mixture of ammonium polyphosphate (APP) and pentaerythritol and (2) surface-modified APP (m-APP) were examined. In terms of steady heat release rate, total heat evolved, and fire growth index determined by mass loss calorimetry, m-APP performed markedly superior to APP-pentaerythritol. The presence of CaCO3 in both intumescent formulations caused significant losses in fire retardant performance assessed by mass loss calorimetry, limiting oxygen index and UL-94 tests. Peak rates of heat release and mass loss during combustion, and total heat evolved on combustion were increased, whereas time to ignition was decreased. Characterization of fire residues ascribed the mechanism of deterioration in fire retardancy to the formation of porous and nonexpanded crystalline calcium phosphate/CaCO3 residues during combustion rather than the amorphous protective intumescent chars formed in the absence of CaCO3. POLYm. ENG. SCI., 51:875-883, 2011. (C) 2011 Society of Plastics Engineers