The copolymer of acrylamide (AM) and 2-acrylamido-2-methyl-1-propane sulfonate (AMPS) was synthesized through the free radical dispersion polymerization in an aqueous solution of ammonium sulfate and in the presence of poly(2-acrylamido-2-methyl-1-propane sulfonate) as stabilizer. The average particle size of the copolymer ranged from 1 to 4 pm, and the molecular weight was from 2.0 X 10(6) to 7.0 X 10(6) g mol(-1). By analyzing apparent viscosity and particle size, the swelling property of the dispersion copolymer was studied. When the dispersion was diluted with salt water in which the ammonium sulfate concentration kept equal with that of the original dispersion, particle size and particle size distribution of the diluted dispersion changed a little, compared with that of the original dispersion. While diluted with deionized water, particle size and particle size distribution could expand several times. The effects of varying concentrations of the stabilizer, the monomer, the salt and the initiator on particle size, and molecular weight of the copolymer were investigated, respectively. The reaction conditions for preparing stable dispersion were concentrations of 20-28% of the salt, 6-14% of monomers, and 1.8-2.7% of the stabilizer. (c) 2006 Wiley Periodicals, Inc.