We have used multinuclear NMR and IR spectroscopy to study the interaction of a number of metal cations with monovacant heteropolyanion [P2W20O70(H2O)(2)](10-) (P2W20) in aqueous solutions starting from its K salt. We have also prepared and studied P2W20 in an Na-only medium. The observed differences in the NMR spectra of NaP2W20 and KP2W20 solutions and the importance of K+ and Na+ for the formation of P2W20 suggest that this polyanion exists only as a complex with the alkaline cations. When both cations were simultaneously present in solution, we observed the broadening of the NMR signals of P2W20 due to the Na-K exchange. Li+ does not replace K+ or Na+ in such complexes, and in an Li-only medium P2W20 does not form. Of all the M"+ cations studied (Pd2+, Bi3+, Sn4+, Z(4+), Ce4+, Ti4+, V5+, and Mo6+) only Bi3+, Sn4+, and Ce4+ form complexes with P2W20 in strongly acidic solutions. The W-183 and Sn-119 NMR data suggest that Sn4+ forms in solution two mutually interconvertable P2W20Sn complexes of the composition P2W20O70(H2O)(3)SnOH7- and (P2W20O70(H2O)(3)Sn)(2)O14- while Bi3+ forms one complex of the proposed composition P2W20O70(H2O)(2)Bi7-. We obtained complexes with Bi and Sn as free heteropoly acids and studied their thermostability in the solid state.