New chelating and antioxidant polymeric microspheres were synthesized through precipitation polymerization of 4-O-(4-vinylbenzyl)-myo-inositol 1,3,5-orthoformate with ethylene glycol dimethacrylate and subsequent exhaustive phosphorylation of the myo-inositol groups with phosphoric acid. Three different microspheres using different molar ratios of the two co-monomers were prepared. The antioxidant properties of these polymers were evaluated in rat liver microsomal membranes. This study showed that the macromolecular systems are very effective at inhibiting iron-dependent lipid peroxidation of the membranes. This antioxidant effect is due to the chelating properties of polyphosphorylated inositol residues in the polymeric devices toward ferric ions. The amount of polyphosphorylated inositol in the microspheres was found to play a crucial role in determining the chelating efficiency of the polymers: the polymer with the greatest amount of polyphosphorylated inositol was the most powerful antioxidant polymer.