Near infrared spectroscopy (NIRS) provides means for real-time monitoring of polymerization reactors. Previous experimental studies report successful implementations of NIR-based monitoring techniques for styrene, vinyl chloride and acrylic acid / vinyl acetate suspension polymerizations; however, no information is available in the literature for methyl methacrylate (MMA) polymerizations. For this reason, the main objective of this paper was analyzing the NIR spectra obtained in-line and in situ during MMA suspension polymerizations and observing whether NIR spectra may be used for in-line monitoring and control of the average particle size in these systems. Particularly, NIR spectral responses to changes of the average particle size were analyzed in poly(vinyl chloride), polystyrene and PMMA suspensions, showing that the NIR spectra of distinct reacting systems respond differently to modification of the suspended particle sizes. Finally, it was observed that the control of average particle sizes and particle size distributions in MMA suspension polymerizations requires that control actions be taken at the initial polymerization stages, when monomer conversion is below 30 wt%.