Polymeric microspheres were synthesized by the precipitation copolymerization of methacrylic acid and styrene in supercritical carbon dioxide. Scanning electron microscopy showed that the products were spherical microparticles. The mean diameter of the particles was 0.2-2 mu m. The synthesis conditions affecting the particle size and morphology were examined in detail. The well-distributed copolymer microspheres were applied as low-temperature improvers for diesel. The results showed excellent performance for high-paraffin diesel at low temperatures. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2749-2753, 2010