Stabilizer-free dispersion copolymerization is a one-step technique to produce uniform polymer particles. Due to its fast kinetics and nucleation, assessing polymer conversion and particle size via off-line sampling can be a challenge. In addition, off-line analyses, e.g., gravimetry and dynamic light scattering, are time-consuming and can be sample-dependent. Near-infrared (NIR) spectroscopy is a fast and non-destructive technique to monitor polymerization reactions in-line. Stabilizer-free dispersion copolymerization between styrene and maleic anhydride was carried out via freeradical polymerization using isobutyl acetate and isopentyl acetate as solvents. Spectra data were collected in parallel with off-line analysis. Multivariate partial least squares calibration models were constructed to predict global conversion and particle size over time. The external validation shows good agreement between the models and experimental data. NIR can be extended to monitor systems with fast kinetics and to avoid destructive and time-consuming analytical methods.