Eudragit E, a copolymer of N,N-dimethylaminoethyl methacrylate and neutral methacrylic acid esters, has been found to form six-membered cyclic anhydrides via intramolecular ester condensation by a heating process only, without adding any catalyst or initiator. A novel reflectance Fourier transform infrared microspectroscopy system equipped with differential scanning calorimetry (FTIR/DSC) was used to determine the thermal-dependent anhydride formation of Eudragit E film, The anhydride-related IR peaks at 1801, 1763 and 1007 cm(-1) appeared from 180 degrees C and increased with the heating temperature, while the peaks at 2820 and 2770 cm(-1) for the dimethylamino group reduced gradually with the increase of temperature, strongly suggesting that the anhydride formation by ester condensation started above 180 degrees C, The appearance of peaks at 1801 and 1763 cm(-1) not only illustrates the asymmetric and symmetric stretching vibration mode of the carbonyl group existing in the anhydride structure, but also exhibits the formation of six-membered cyclic anhydrides during the heating process. Moreover, the higher peak intensity ratio of 1763 cm(-1)/1801 cm(-1) also reveals the predominant role of intramolecular ester condensation in anhydride formation in Eudragit E film.