scintillation microspheres (PSm) are a novel material employed in the measurement of radioactivity (α and β emitters). This work is focused on the formation of plastic scintillation particles through the precipitation and encapsulation of two fluorescent solutes (2,5-diphenyloxazol (PPO) and 1,4-Bis(5-phenyloxazol-2-yl) benzene (POPOP)) and an aromatic solvent 2,6-diisopropyl-naphthalene (DIN), which acts as an enhancer for α and β emitters discrimination, into a polymeric matrix of Polystyrene (PS) by Supercritical Anti-Solvent process (SAS) using ethyl acetate (EtAc) for dissolving the PS and supercritical CO2 as antisolvent. Different process parameters were varied; solute concentration in the organic solution (W, wt%), injection velocity of the organic solution (u, ms−1), molar ratio of the organic solvent regarding to CO2 (XEA) and injection capillary tube diameter (μm). In the different experimental conditions tested, SAS coprecipitation was successfully achieved resulting in yields higher than 90% and very low quantities of residual solvent (600–1200 ppm). The different Polystyrene based particles obtained were nearly spherical sub-micron particles (ranged between 150 and 400 nm) and also agglomerates of a few micrometers were observed in most of the studied conditions. Radiometric capacities of particles were evaluated through measuring different beta and alpha emitting radionuclide. The coprecipitates showed scintillation behavior when fluorescent solutes were added, therefore confirming their encapsulation.