Waste ABS (acrylonitrile-butadiene-styrene) resin and waste AS (acrylonitrile-styrene) resin used for videocassettes were converted into a water-absorbent polymer. Concretely, these waste resins containing acrylonitrile were made to react with heated high-concentration sulfuric acid. In this chemical modification, the acrylonitrile portion and the styrene portion in the AS resin were hydrolyzed and sulfonated, respectively. We studied the relationship between the water absorbency of the modified waste AS and the conditions of chemical modification. It was confirmed that the water absorbency greatly depends on the degree of hydrolysis of the acrylonitrile portion in the waste AS resin. We have succeeded in developing a water-absorbent polymer, which can absorb water of about 800 times of its own weight by the chemical modification of the waste AS resin. (c) 2008 Wiley Periodicals, Inc.