Micronized SrSO4 crystals having monodispersed/narrow distributions were produced with a controlled double-jet reactive crystallizer. Polyethylenimine (PEI) had the role of a suitable controller of supersaturation and following crystallization phenomena, and then led to agglomeration-less homogeneous crystalline qualities. Especially, the PEI imines seemed to adjust the number of nucleates, and also physical gel networks seemed to contribute to the inhibition of further particulate agglomeration. The current paper mainly demonstrated the influence of PEI addition on the resulting properties, e.g. sizes and size distribution widths. Therefore the polyelectrolyte-assisted technique will develop as a build-up process contributing to provide high-value nano/micro-powders having the desired crystalline qualities.