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The fire performance of polystyrene‐organic montmorillonite (OMMT) nanocomposite was investigated by limiting oxygen index (LOI) and cone calorimetry. Scanning electron microscopy, electron dispersive spectroscopy and attenuated total reflection Fourier transform infrared spectroscopy were employed to study the charring process of the nanocomposite. The residue collected upon thermal degradation was analyzed by various means to determine its composition and to understand the flame‐retardant mechanism of the nanocomposite. It has been shown that the introduction of OMMT does not have much influence on LOI of the nanocomposite, but can greatly decrease the heat release rate (HRR) and mass loss rate (MLR) and enhance the flame retardancy of the material. The flame‐retardant mechanism is due to charring in the condensed phase. The intercalated nanostructure is destroyed, and the silicate nanolayers in the nanocomposite rearrange and accumulate on the material surface during pyrolysis. The charred residue has a honeycomb‐like porous structure, which covers on the material surface and serves as a protection barrier against heat transfer and mass exchange, leading to enhanced flame retardancy. The charred residue is composed of pyrolyzed silicate layers and graphitic char. The char is highly stable in nitrogen even at 800 °C, but thermo‐oxidative decomposition is allowed, and it can be removed completely in the presence of air. Due to the porous structure of the charred residue, the protection from it is mainly to reduce the HRR and MLR and retard burning of the material. It is not enough to make the nanocomposite self‐extinguish. Copyright © 2012 John Wiley & Sons, Ltd.

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