The reaction of [P2W15O56](12-) and aqueous Fe3+ forms two ferric polytungstophosphate derivatives that have been isolated, purified, and characterized by X-ray crystallography and several physical methods. At pH similar to 3 and [Na+]=2 M, needles of Na-12[Fe-4(III)(H2O)(2)(P2W15O56)(2)].58H(2)O, Na-1, form, in which the conventional ion packing arrangement in solid heteropoly acids is observed: each polyanion, [Fe-4(III)(H2O)(2)(P2W15O56)(2)](12-) (1), is well separated by the cations (Na+). At pH similar to 1 and [Na+]=1 hi, however, elongated prisms of H3Na9[Fe-4(III)(H2O)(2)-(P2W15O56)(2)].62H(2)O, (Na-l)(n), form. (Na-l), is a novel one-dimensional inorganic polymer composed of 1 linked via ion pairing with strongly associated hydrated sodium cations. Both Na-1 and (Na-1)(n) display identical characteristics in H2O solution. Seven lines of evidence are consistent with the Fe-4(III) oxidation state in 1: (1) the synthesis itself (only Fe3+ used); (2) bond-length-based valence sum calculations based on the two X-ray structures; (3) the electrochemical behavior; (4) the charge requirements and elemental analyses; (5) no reaction between 1 and Ce4+; (6) the lack of a Jahn-Teller distortion; and (7) the infrared spectrum relative to those of the literature [M-4(H2O)(2)(P2W15O56)(2)](16-) sandwich compounds, where M=a divalent first-row transition metal ion in all cases. The tetra-n-butylammonium salt of 1, TEA-1, exhibits catalytic activity for oxidation of alkenes by H2O2. The catalytic effect is substantial. For example, in the reaction of cyclohexene + H:O:, rate(with TBA-l)/rate(without TEA-1)(-d[cyclohexene]/dt) is similar to 25. Alkene oxidation is characterized predominantly by allylic attack on aliphatic substrates and oxidative cleavage of stilbenes. The polyanion, 1, is quite stable under catalytic conditions (no decomposition of 1 was detected by P-31 NMR after a solution of 1, alkene, and 0,25 M H2O2 was incubated for 48 h).