Catalytic hydrolysis of toxic organic environmental contaminants such as organophosphates by inorganic nanocrystals or polymeric latexes is emerging as a promising alternative to other remediation approaches. We have recently developed a novel catalytic medium, an inorganic nanoparticle surface-modified by a reactive polymer that can be considered a hybrid between nanocrystals and latexes. In the present work, a model warfare agent, diisopropyl fluorophosphate, is hydrolyzed catalytically in aqueous media by suspensions of magnetite (Fe3O4) nanoparticles modified with poly(1-vinylimidazole-co-acrolein oxime-co-acrylic acid) that acts in an enzyme-like fashion. The oxime-and imidazole-modified magnetite particle serves as a nanosized particulate carrier with nucleophilic groups immobilized on its surface. The oxime-modified magnetite nanoparticles are colloidally stable within a wide pH range and are readily recovered for reuse from the aqueous milieu by high-gradient magnetic separation methods with no loss of catalytic activity. Because of the positive charge at neutral pH, the novel particles adsorb organophosphoric acids, the products of the nerve agent decomposition, thus affording a one-step water remediation.