The thermal stability of the powder or film forms of Eudragits L, S, and L30D and carbopol polymers was qualitatively investigated by microscopic reflectance Fourier transform infrared spectroscopy equipped with differential scanning calorimetry (FT-IR/DSC microscopic system). DSC and thermogravimetric analysis (TGA) were also used to study the thermal behavior of the pulverized powders of the raw material and the films of these polymers. The results indicate that a 6-membered ring cyclic anhydride formation was respectively produced by a heating process in the molecular structures of Eudragits L, S, and L30D and carbopol polymers via inter-or intrapolymer condensation. The peak intensity of the carbonyl stretching absorption of cyclic anhydride-related IR peaks appeared sharply near 1801 and 1759 (1751) cm ฯช1 , and a peak near 1006 (1018) cm ฯช1 related to the antisymmetric stretching mode of COOOC also appeared suddenly beyond the marked reaction temperature. The initial reaction temperature of anhydride formation for the Eudragits L, S, and L30D and carbopol polymers was observed respectively from near 186, 180, 170, and 100ยฐC, whereas the rapid and marked reaction temperature from about 206, 188, 170, and 183ยฐC appeared accurately from the reflectance FT-IR spectra. The DSC thermograms and TGA curves also confirmed these results.