Once the sample was continuously reheated from 100 to 130°C (Fig. 3d→Fig. 3e), its IR spectrum was still so sharp and had shifting behavior at several frequencies. The peaks at 3338cmϪ1 assigned to N–H stretching, at 1651cmϪ1 due to the CϭO stretching, at 1550cmϪ1 corresponding to N–H bending, and at 1319 cmϪ1 assigned to O–H bending shifted to 3334, 1660, 1558 and 1323cmϪ1, respectively. Because the IR spectra for 100 °C-heated and 130 °C-heated samples were so sharp and different from each other, they should be of a different polymorph. The thermal change near 118 °C might be due to the polymorphic phase transition. Since the reheated acetaminophen in KBr disc showed a melting point at 153 °C determined by thermal FT-IR microscopic study, near the melting point (159 °C) of form II acetaminophen de- termined by DSC analysis (Fig. 4), we supposed that the 130°C-heated sample in KBr disc should be the form II ac- etaminophen with a melting point at 153°C (Figs. 1B, 2B).