Micronized BaSO4 particles with a narrow size distribution were produced by using a double-jet reactive crystallizer. Polyethylenimine (PEI) acted as a growth/agglomeration inhibitor at a high PEI dosage, hence the particle size and size distribution width decreased, and BaSO4 crystals with the desired qualities were obtained. Similar results were obtained in previous studies in which of SrSO4 and PbSO4. We believe that our method, which involves the use of PEI, can be employed for the production of various inorganic sulfates in which the particles have a monodisperse size distribution.