Abstract  The polymer nanocomposites polystyrene (PS)/kaolinite (Kao) were synthesized by in situ emulsion polymerization. First Kao was modified using dimethylsulfoxide (DMSO) by a displacement method (Kao-DMSO) and then styrene monomer was intercalated into kaolinite. Two types of nanocomposites were synthesized using 1 and 3 % weight (wt) of Kao-DMSO, respectively, (based on the monomer). The comparison between TGA analyses of nanocomposites with 3 % Kao-DMSO and virgin PS revealed that the thermal decomposition of the nanocomposites with clay began at 20oC above the temperature of the initial thermal decomposition of virgin PS. This result showed the effect of clay on the thermal stability of PS. The influence of 3 % Kao-DMSO clay on nanocomposites was also observed during a flame retarding test, in which the burning rate was reduced by 50 % when compared to the burning rate for virgin PS.