The initial oligomerization of bis(hydroxyethyl terephthalate), BHET, was monitored insitu by an attenuated total reflection (ATR)/infrared (IR) probe. The progress of the reaction can be probed by the intensity changes in IR bands, and calibration curves for the degree of oligomerization can be developed on the basis of these spectral changes. It was found that the calibration models can be improved significantly using partial least-squares (PLS) regression. The amount of hydroxyl end group, which is a function of the degree of the polymerization, was measured by use of IH NMR. This parameter was also used to construct the calibration models. The amount of ethylene glycol (EG) in the reaction solution was predicted by the same method. These parameters were found to be very important in understanding the polymerization reaction, From the degree of oligomerization and concentration of flee EG, one may deduce the reaction rate constants. Conventional methods for monitoring the reaction involve manual samplings, so that it is both tedious and dangerous to collect enough data. The on-line method like the present one can solve these issues.