In a previous paper, Santos et al. (J Appl Polym Sci 1998, 20, 1737) showed that NIRS may be used efficiently for in-line evaluation of average particle sizes in styrene suspension polymerizations if proper calibration is carried out with the help of both multivariate techniques and nonlinear models. In the present work, the technique presented by Santos et al. was used for in-line evaluation and for control and design of average particle sizes during styrene suspension polymerizations carried out in the batch mode. The effects of agitation speed and stabilizer concentration on the particle-size Distribution (PSD) were investigated. It is shown here that this technique allows the successful design and real-time control of particle sizes in lab-scale styrene suspension polymerization reactors.