This study presents a thorough qual.-quant. anal. by self-modeling curve resoln. (SMCR) methods of ATR/FT-IR spectra of the polycondensation reaction of bis(hydroxyethylterephthalate) (BHET) that yields poly(ethylene terephthalate) (PET) as a final product. We used five different SMCR methods to ext. pure-component spectra and concn. profiles. All the results obtained by these five methods are compared. It is shown that particular attention should be given to the scaling of the data. The spectral profiles of the species are obtained through the pure variables detd. Because of the appearance of neg. absorbances, the spectra are subjected to the alternating least-square procedure. Rank anal. of the mean centered data shows that the IR spectra of the system in the reaction vessel are predominantly detd. by those of BHET and PET, but that a small amt. of ethylene glycol (EG), which was evacuated from the reaction vessel, also contributes to the spectra.