A near-IR spectroscopic technique for cure characterization of unsaturated polyester resin has been developed, first by using model compound mixtures to identify and quantify various reactions and then by applying these quantification techniques to the resin, with IR spectroscopy as a complementary technique. Analysis of NIR spectra of model compound mixtures (styrene/ethylbenzene, diethyl fumarate/ diethyl succinate) showed that the integrated peak areas at 1629 and 2087 nm could be used to quantify the changes in the concentration of styrene and vinylene C=C bonds in the unsaturated polyester resin, respectively. NIR-based conversions were compared with those values using mid-IR spectroscopy. Differences in NIR and mid-IR conversion values have been explained by the sample volume and heat buildup differences in the samples used to obtain the NIR and mid-IR measurements. NIR results also confirmed an optimum temperature for the highest overall conversion and vinylene conversion, probably due to the reduced diffusion of styrene into tightly cross-linked microgels at high temperature. The complex trend of styrene conversion as a function of vinylene conversion at 75 degreesC, analyzed by NIR, can be approximated by taking into account the reactivity ratios in the copolymerization reaction.