Uniform polystyrene (PSt) particles with the size of 1.9 mu m were first prepared via dispersion polymerization, and then used as the seeds in a second-stage dispersion copolymerization of styrene (St) and methacrylic acid (MAA) to produce carboxyl-carrying microspheres. The PSt seed particles were swollen by monomer mixture of St and MAA, including an oil-soluble initiator 2,2'-azobisiso-butyronitrile (AIBN), before polymerization. Finally, uniform PS/P(St/MAA) (polydispersity index, PDI = 1.02) microspheres with the size of 2.2 mu m were obtained. The average particle size and size distribution of the final microspheres were investigated. MAA contents between 54 and 97 mg/g were detected from the PS/P(St/MAA) particles produced under different conditions. Dispersion medium has great influence on the kinetics of polymerization, due to its effect on the partitioning of monomers, solvents, and initiator in the particle phase, probably as well as on the conformation of the dispersion agent on the surface of the particles. (c) 2006 Wiley Periodicals, Inc.