The initial oligomerization of bis(hydroxyethyl terephthalate), BHET, was monitored in situ at 220, 250, and 270 degreesC by attenuated total reflection (ATR)/infrared (IR) and near-infrared (NIR) transmission spectroscopy. The obtained sets of IR and NIR spectra, which include information about the progress of the reaction, were analyzed by means of generalized two-dimensional (2D) correlation spectroscopy. Both 2D IR and 2D NIR correlation spectra clearly identified bands at 1030, 880, and 861 cm(-1) (IR region) and 5830 cm(-1) (NIR region) arising from ethylene glycol (EG), which is expelled out of the system during the reaction. The synchronous hetero 2D NIR-IR correlation spectrum showed positive cross-peaks between the IR peaks at 1030, 880, and 861 cm(-1) and the NIR band at 5830 cm(-1) while no cross-peak appeared in the corresponding asynchronous spectrum, confirming that all these peaks arise from the same origin (EG). To carry out the 2D hetero correlation analysis, the experiment was modified so that both ATR/IR and NIR probes could be employed simultanously. The 2D NIR correlation ana-lysis also suggested that a band at 5260 cm(-1) is ascribed to water. It was found that NIR spectroscopy may be very useful to in situ monitor the amount of water in the reaction system.