The innovative characteristics of microreactors allow producing new materials. The present work analyses the behavior of the Syrris ASIA millireactor for the solution polymerization of styrene. The polymerizations were performed at 100°C, using benzoyl peroxide as initiator and toluene as solvent. Different monomer to solvent ratios and initiator quantities have been tested, besides varying the residence time from 5 to 80 minutes. The variations of the parameters involved gave conversions from 9% to 70%, with MW ranging from 6,000 to 60,000 g/mol. No plugging happened, allowing more aggressive and well controlled applications. A first attempt of modeling was made, using a PFR model and the method of moments to compute the mean distribution. This simplified model gave a good forecast of conversion and average MW for the more diluted experiments, but partially deviated for higher monomer contents, with larger PDIs, meaning a greater discrepancy from plug flow for the millireactor.