The syntheses, crystal structures, and physical properties of the series of radical salts made with bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) and monosubstituted alpha-Keggin polyoxoanions of formula [XZ(H2O)M11O39](5-)(XZM(11) = (SiFeMo11)-Fe-IV-Mo-III, (SiCrW11)-Cr-IV-W-III, (PCoW11)-Co-V-W-II, (PNiW11)-Ni-V-W-II, (PCuW11)-Cu-V-W-II (PZnW11)-Zn-V-W-II, (PMnW11)-Mn-V-W-II, and (PMnMo11)-Mn-V-Mo-II) containing a magnetic metal ion Z on a peripheral octahedral site of the Keggin anion are reported. They all crystallize in two related series called alpha(2) and alpha(3). The general structure consists of alternating layers of the organic donor and the Keggin polyoxometalates. While the stoichiometry and alpha-packing arrangement within the organic ET layers are the same in the two phases, significant differences are observed in the anion layers. Thus, in the alpha(3) phase the anions polymerize to form an unprecedented linear chain of Keggin anions along the c axis, while in the remaining compounds the anion layers are formed by discrete Keggin units. A salient feature of these structures is that in both phases the magnetic metal ion Z appears to be localized on two of the 12 possible octahedral sites of the monosubstituted Keggin anion, even if the Keggin anions are not bonded, as in the alpha(2) phase. The organic layers are formed by two different kinds of stacks : an eclipsed chain with almost totally ionized ET molecules and a dimerized one with partially charged ET molecules that accounts for the semiconducting character of these salts. Magnetic measurements of the radical salt with the diamagnetic Zn-containing Keggin anion indicate the presence of antiferromagnetic interactions in the organic sublattice (/J/ approximate to 70 cm(-1)), together with a Curie tail at low temperatures, which has been attributed to a paramagnetic contribution coming from the progressive electron localization in the mixed-valence dimerized chain when cooling. All the salts having discrete magnetic anions (alpha(2) phase) show magnetic behaviors which correspond to the sum of the magnetic contributions of the two sublattices. This observation indicates that the interactions between the two sublattices are quite negligible, a result in agreement with the EPR measurements. However, with the two radical salts having chains of Keggin anions (alpha(3) phase : PMnW11 and PMnMo11) small differences between the ET and the tetrabutylammonium salts have been detected at low temperatures. In the [PMnW11] derivative these differences have been attributed to single-ion effects. In the [PMnMo11] derivative, these differences have been attributed to a weak antiferromagnetic Mn-Mn interaction promoted by the presence of a delocalized electron on the mixed-valence polyanion.