Fiber-optic near-infrared (NIR) spectroscopy was used to monitor the monomer conversion and the weight-average molecular weight of the polymer produced during solution polymerization of methyl methacrylate (MMA) carried out in a lab-scale reactor. NIR spectra were recorded during batch and semi-continuous reactions using an in situ transmission probe. Off-line gravimetry and GPC were used as reference methods to provide the conversion and the average molecular weight data set required for the calibration procedure. A statistical model was generated using partial least-squares regression (PLS) to relate the NIR spectral data to the two polymerization variables of interest. The measurements were then validated for various operating conditions (i.e., different solvent, initiator, MMA, and chain-transfer agent concentrations) and for both batch and semi-continuous modes. The conversion was predicted during three validation experiments with an average standard error of prediction (SEP) of 2.1%. The on-line evaluation of M-w was obtained with an average relative SEP of 5.7%; such on-line NIR measurement was thus demonstrated to be robust and accurate, even in the case of versatile use of the polymerization plant.