The influence of the nature of alkali metal cations on the structure of the species obtained from the trivacant precursor A-alpha-[SiW9O34](10-) has been studied. Starting from the potassium salt 1, K(10)A-alpha-[SiW9O34].24H(2)O, the sandwich-type complex 2, K-10.75[Co(H2O)(6)](0.5)[Co(H2O)(4)Cl](0.25)A-alpha-[K-2{Co(H2O)(2)}(3)(SiW9O34)(2)].32H(2)O, has been obtained. The crystal structures of these two compounds consist of two A-alpha-[SiW9O34](10-) anions linked by a set of potassium (1) or cobalt plus potassium cations (2), and the relative orientation of the two half-anions is the same. Attempts to link two A-alpha-[SiW9O34](10-) anions by tungsten atoms instead of cobalt failed whatever the alkali metal cation. Moreover, the nondisordered structure of Cs-15[K(SiW11O39)(2)].39H(2)O is described. Two [SiW11O39](8-) anions are linked through a potassium cation with a "trans-oid" conformation, and the potassium occupies a cubic coordination site.