Nanostructured CdS and ZnS have been synthesized by chemical precipitation method using DMF as stabilizing agent. Nanocomposites of conducting polyaniline with CdS/ZnS nanoparticles have been synthesized by chemical oxidation polymerization method using ammonium persulphate (oxidant) and sulfuric acid (dopant). A series of the nanocomposites have been synthesized by varying the weight percentage of CdS/ZnS (5, 10, 15, 20%) in the polymerization of aniline (monomer). Crystalline nature and size of the particles have been analyzed by X-ray diffraction study. The particle size was found to be less than 10 nm which has been confirmed from TEM images. Thermal stability of polymer nanocomposites has been analyzed by TGA and corresponding thermal kinetic parameters were calculated. As the weight percentage of CdS/ZnS nanoparticles increases in the polyaniline (PANI) matrix, half decomposition temperature increases gradually and activation energy also increases. These results indicate that the polymer nanocomposites are thermally more stable as compared to that of pure PANI.