Poly(acrylonitrile-co-methyl methacrylate) [P(AN-MMA)] copolymer was obtained in the presence of 2,2'-azobisisobutyronitrile by using acrylonitrile and methyl methacrylate monomers via free radical polymerization. The surface of the synthesized copolymer was hydrolyzed in sodium hydroxide solution. Nitrile groups (-C≡N) on the P(AN-MMA) copolymer surface were converted into conjugated imine groups (-C=N-). Hydrolyzed poly(acrylonitrile-co-methyl methacrylate) [HP(AN-MMA)] copolymer was used in the adsorption of magnesium ions. Magnesium ion was determined with ethylenediaminetetraacetic acid in a water solution containing Mg(NO3)2.6H2O before and after adsorption. The adsorption capacity of the HP(AN-MMA) copolymer against Mg2+ ions was found to be 1.27 mmol.g-1 (pH=4.5). After isolation and drying of the product, its structural and thermal characterization was determined by 1H-nuclear magnetic resonance spectroscopy, Fourier transform-infrared spectroscopy and thermogravimetric analysis, which are the basic polymer characterization methods. The average surface roughness (Ra) values developed after magnesium ion adsorption were investigated by atomic force microscopy analysis. With magnesium ion adsorption, the Ra value increased from 136 nm to 354 nm.