Three-dimensional plots of FT-IR spectra of acetaminophen on first-heating (A) and second-heating (B) with respect to the temperature are shown in Fig. 1. Obviously, the contour IR profile of aceta- minophen on first-heating changed dramatically near 165 °C (Fig. 1A), which was close to the melting point (168— 170 °C) of acetaminophen.4,6,10,13)