Several new large polyoxotungstates have been synthesized by reaction of lanthanide cations with the well-known "As4W40" anion, [(B-alpha -AsO3W9O30)(4)(WO2)(4)](28-) (1). The heteropolyanions [(H2O)(11)Ln(III)(Ln(III2)OH)(B-alpha AsO3W9O30)(4)(WO2)(4)](20-) (Ln = Ce, Nd, Sm, Gd) (2-4) (Ln(3)As(4)W(40)) and [M-m(H2O)(10)(Ln(2)(III)OH)(2)(B-alpha -AsO3W9O30)(4)(WO2)(4)]((18-m)-) (Ln = La, Ce, Gd and M = Ba, K, none) (5-7) (Ln(4)AS(4)W(40)) have been isolated as alkali metal and ammonium salts, respectively, and characterized by single-crystal X-ray analysis, elemental analysis, and IR and W-183-NMR spectroscopy. The X-ray analyses revealed interanionic W-O-Ln bonds between adjacent Ln(x)As(4)W(40) units forming a "dimer" for x = 3 and chains for x = 4. Upon dissolving in water these bonds hydrolyze and the monomeric species form. The straightforward syntheses which require the use of concentrated NaCl solutions (1-4 M) and the addition of stoichiometric amounts of Ba2+ or K+ reemphasize the importance of the presence of appropriate countercations for the assembly of large polyoxometalate structures.