Organophosphorus pesticides and warfare agents are not readily hydrolyzed in aqueous media without applying extremes of pH, heat, or bleach. We show that suspensions of magnetite (Fe3O4) nanoparticles modified with a common antidote, 2-pralidoxime (PAM) or its polymeric analogue, poly(4-vinylpyridine-N-phenacyloxime-co-acrylic acid), catalyze the hydrolysis of an organophosphate (OP) compound serving as a model of the warfare nerve agents, at neutral pH. The oxime-modified magnetite particle serves as a nanosized particulate carrier with a powerful alpha-nucleophile, the oximate group, immobilized on its surface. The rates of OP hydrolysis by the PAM-modified magnetite are comparable to those of the most potent copper-based catalysts. The oxime-modified magnetite nanoparticles are colloidally stable at neutral pH and are readily recovered for reuse from the aqueous milieu by high-gradient magnetic separation methods with no loss of catalytic activity.