A novel reflectance Fourier transform infra-red/differential scanning calorimetry (FTi.r./d.s.c.) microscopic system was used to determine the temperature-dependent anhydride formation of Eudragit L-100 film. A three-dimensional plot of the reflectance FTi.r. spectra of Eudragit L film with respect to temperature was obtained. With the increase of heating temperature, three bands at 1804, 1763 and 1006 cm(-1) suddenly appeared beyond 206 degrees C and these peak intensities became markedly stronger. The appearance of these peaks during heating might be attributed to the anhydride formation by inter- or intrapolymer condensation, since acid anhydrides give two carbonyl stretching vibrations at 1800-1750 cm(-1) and one antisymmetric C-O-C stretching band.