In this work, the sensitivity of NIR spectroscopy was studied for in-line, in-situ monitoring of residual monomer concentrations in solution copolymerization of styrene and divinylbenzene (S-DVB). Reactions were carried out in a glass reactor (stirred tank) to gather data used to develop a NIR calibration model. Experimental data were collected via off-line measurements of monomer conversion by gravimetry and they were previously validated with a kinetic model of the process. These data were correlated with the NIR spectra collected by an immersion probe (transflectance mode) during the reaction at the same sampling times. Spectral changes in the ranges 4460-4750 cm -1 and 6212-6187 cm -1 were related to the variations of monomer concentration. The results show good agreement between the experimental data and in-line monitoring predictions from the NIR spectra and the calibration model.