The aim of this work is to investigate the production of polypropylene microparticles from polypropylene/n-butane solutions using a modified rapid expansion of supercritical solutions (RESS) technique. A commercial polypropylene sample (Polibrasil Resinas S.A.) produced from a Ziegler-Natta catalyst was employed in the experiments. The experiments were performed in a laboratory scale unit where the effects of pre-expansion temperature (373-423 K), polymer concentration in the solution (0.1-1.0 wt.%) and the ratio of length to capillary diameter (20-100) were evaluated. Scanning electronic microscopy was employed to analyze the morphology and size of the produced particles. Results show that polymer concentration was the variable that exerted a more pronounced effect on the morphology of the precipitated particles. Besides, it is shown that the proposed technique can be used to transform polypropylene particles with very broad size distributions into a variety of products, ranging from interconnected fibers to monodisperse micro-spheres, through manipulation of the operation variables of the expansion process. (c) 2004 Elsevier B.V. All rights reserved.