Different formulations were designed to evaluate the effect of organically modified clay (DK4) on the combustion behavior of polystyrene (PS) containing an intumescent flame retardant, poly(4,4-diaminodiphenyl methane spirocyclic pentaerythritol bisphosphonate) (PDSPB). The results of transmission electron microscopy reveal that DK4 selectively dispersed in the PDSPB phase. An investigation of thermogravimetric analysis revealed that the thermal stability of PS resin showed no obvious change with the addition of PDSPB and DK4, but the residue increased. From the results of cone calorimetry, we observed that there were two steps during combustion. The dispersion of DK4 played an important role in improving the thermal stability and the flammability of the PS/PDSPB/DK4 nanocomposites. In the first step, DK4 was restricted in the PDSPB phase; there was no synergistic effect. A synergistic effect occurred in the second step when clay had a homogeneous distribution, in which the peak heat release rates were reduced by about 40 and 61% compared to the pure PS. A model of combustion behavior was developed according to these results. The synergistic mechanism was caused by the formation of the silicoaluminophosphate (SAPO) structure formed by reactions between PDSPB and DK4. Field emission scanning electron microscopy characterization showed that such an SAPO structure led to a ceramic-like residue after burning. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 777-783, 2010