The effect of pH value on the ultrasonic copolymerization of polyvinyl alcohol (PVA) in its aqueous solution with acrylonitrile (AN) was studied. It was found that the reaction might be controlled merely by varying the pH value of the system. The resulted irradiation mixture was a stable water based dispersion. The particle size of the copolymer formed increased with the decrease of pH value. With the decrease of pH value
both the yield of copolymer and AN content in the copolymer would be increased. No homopolymer of AN was found in the reaction products.