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 20 °C 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 Ver menos