Soap-free emulsion copolymerization of styrene and acrylonitrile was carried out by use of potassium persulfate-sodium, bisulfite redox initiator. Effects of initiator concestraction, monomer to water ratio, monomer feed ratio and polymerization temperature on the rate of polymerization were investigated. It was found that the conversion increased with initiator concentration, the ratio of monomer to water, the concentration of acrylonitrile in the feed and polymerization temperature. The overall activation energy for the copolymerization was determined to be 11.8 ㎉/mole from the Arrhenius plot. The content of acrylonitrile in the copolymers increased with the ratio of monomer to water and the concentration of acrylonitrile in the monomer mixture.