Magnetite (Fe3O4) nanoparticles were synthesized and immobilized in a synthetic resin polymethyl methacrylate (PMMA). The Fe3O4 nanoparticle-PMMA composites were studied for their efficiencies of removing dissolved arsenic (As) and antimony (Sb). The effects of major environmental and operating parameters on the removal of As and Sb were investigated in batch experiments. Singular and competitive adsorption of As and Sb onto the composites were studied. The results demonstrated the capability of the Fe3O4-PMMA composites for removing dissolved metalloids.