The effects of the various salts onto the emulsifier-free emulsion polymerization of methyl methacrylate (MMA) were investigated. It was found that the kind and amount of the salts were very effective on the polymerization even onto the polymeric products. It is known that the ionic strength of the electrolyte is effective for the polymerization. However, our investigation with the same ionic strength of different electrolyte produced the different effect on the polymerization. It was found that the Stokes radiuses of ions are very important for the reaction kinetics and type of the product. At the same electrolyte concentration, as increased Stokes radii of cation of salts that not react (the fragments of initiator, ions, etc.) in polymerization, polymerization rate and average molecular weight of polymer decreased, polymer particle diameter increased. In the case of Br- and SO4= the anions of the salt are also demonstrated some unexpected reactions. (c) 2006 Wiley Periodicals, Inc.