A series of polystyrenes with phosphonicacid (5) vialongalkylsidechains (4, 6, and 8 methylene units) were prepared by the radical polymerization of the corresponding diethyl ω‐(4‐vinylphenoxy)alkylphosphonates, followed by the hydrolysis with trimethylsilyl bromide. The resulting phosphonated polystyrenemembranes had a high oxidative stability against Fenton's reagent at room temperature. The